This patch fixs to do sanity check with user_block_count.

- Overview
Divide zero in utilization when mount() a corrupted f2fs image

- Reproduce (4.18 upstream kernel)

- Kernel message
[  564.099503] F2FS-fs (loop0): invalid crc value
[  564.101991] divide error: 0000 [#1] SMP KASAN PTI
[  564.103103] CPU: 1 PID: 1298 Comm: f2fs_discard-7: Not tainted 4.18.0-rc1+ #4
[  564.104584] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
[  564.106624] RIP: 0010:issue_discard_thread+0x248/0x5c0
[  564.107692] Code: ff ff 48 8b bd e8 fe ff ff 41 8b 9d 4c 04 00 00 e8 cd b8 ad ff 41 8b 85 50 04 00 00 31 d2 48 8d 04 80 48 8d 04 80 48 c1 e0 02 <48> f7 f3 83 f8 50 7e 16 41 c7 86 7c ff ff ff 01 00 00 00 41 c7 86
[  564.111686] RSP: 0018:ffff8801f3117dc0 EFLAGS: 00010206
[  564.112775] RAX: 0000000000000384 RBX: 0000000000000000 RCX: ffffffffb88c1e03
[  564.114250] RDX: 0000000000000000 RSI: dffffc0000000000 RDI: ffff8801e3aa4850
[  564.115706] RBP: ffff8801f3117f00 R08: 1ffffffff751a1d0 R09: fffffbfff751a1d0
[  564.117177] R10: 0000000000000001 R11: fffffbfff751a1d0 R12: 00000000fffffffc
[  564.118634] R13: ffff8801e3aa4400 R14: ffff8801f3117ed8 R15: ffff8801e2050000
[  564.120094] FS:  0000000000000000(0000) GS:ffff8801f6f00000(0000) knlGS:0000000000000000
[  564.121748] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  564.122923] CR2: 000000000202b078 CR3: 00000001f11ac000 CR4: 00000000000006e0
[  564.124383] Call Trace:
[  564.124924]  ? __issue_discard_cmd+0x480/0x480
[  564.125882]  ? __sched_text_start+0x8/0x8
[  564.126756]  ? __kthread_parkme+0xcb/0x100
[  564.127620]  ? kthread_blkcg+0x70/0x70
[  564.128412]  kthread+0x180/0x1d0
[  564.129105]  ? __issue_discard_cmd+0x480/0x480
[  564.130029]  ? kthread_associate_blkcg+0x150/0x150
[  564.131033]  ret_from_fork+0x35/0x40
[  564.131794] Modules linked in: snd_hda_codec_generic snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm snd_timer snd mac_hid i2c_piix4 soundcore ib_iser rdma_cm iw_cm ib_cm ib_core iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx raid1 raid0 multipath linear 8139too crct10dif_pclmul crc32_pclmul qxl drm_kms_helper syscopyarea aesni_intel sysfillrect sysimgblt fb_sys_fops ttm drm aes_x86_64 crypto_simd cryptd 8139cp glue_helper mii pata_acpi floppy
[  564.141798] ---[ end trace 4ce02f25ff7d3df5 ]---
[  564.142773] RIP: 0010:issue_discard_thread+0x248/0x5c0
[  564.143885] Code: ff ff 48 8b bd e8 fe ff ff 41 8b 9d 4c 04 00 00 e8 cd b8 ad ff 41 8b 85 50 04 00 00 31 d2 48 8d 04 80 48 8d 04 80 48 c1 e0 02 <48> f7 f3 83 f8 50 7e 16 41 c7 86 7c ff ff ff 01 00 00 00 41 c7 86
[  564.147776] RSP: 0018:ffff8801f3117dc0 EFLAGS: 00010206
[  564.148856] RAX: 0000000000000384 RBX: 0000000000000000 RCX: ffffffffb88c1e03
[  564.150424] RDX: 0000000000000000 RSI: dffffc0000000000 RDI: ffff8801e3aa4850
[  564.151906] RBP: ffff8801f3117f00 R08: 1ffffffff751a1d0 R09: fffffbfff751a1d0
[  564.153463] R10: 0000000000000001 R11: fffffbfff751a1d0 R12: 00000000fffffffc
[  564.154915] R13: ffff8801e3aa4400 R14: ffff8801f3117ed8 R15: ffff8801e2050000
[  564.156405] FS:  0000000000000000(0000) GS:ffff8801f6f00000(0000) knlGS:0000000000000000
[  564.158070] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  564.159279] CR2: 000000000202b078 CR3: 00000001f11ac000 CR4: 00000000000006e0
[  564.161043] ==================================================================
[  564.162587] BUG: KASAN: stack-out-of-bounds in from_kuid_munged+0x1d/0x50
[  564.163994] Read of size 4 at addr ffff8801f3117c84 by task f2fs_discard-7:/1298

[  564.165852] CPU: 1 PID: 1298 Comm: f2fs_discard-7: Tainted: G      D           4.18.0-rc1+ #4
[  564.167593] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
[  564.169522] Call Trace:
[  564.170057]  dump_stack+0x7b/0xb5
[  564.170778]  print_address_description+0x70/0x290
[  564.171765]  kasan_report+0x291/0x390
[  564.172540]  ? from_kuid_munged+0x1d/0x50
[  564.173408]  __asan_load4+0x78/0x80
[  564.174148]  from_kuid_munged+0x1d/0x50
[  564.174962]  do_notify_parent+0x1f5/0x4f0
[  564.175808]  ? send_sigqueue+0x390/0x390
[  564.176639]  ? css_set_move_task+0x152/0x340
[  564.184197]  do_exit+0x1290/0x1390
[  564.184950]  ? __issue_discard_cmd+0x480/0x480
[  564.185884]  ? mm_update_next_owner+0x380/0x380
[  564.186829]  ? __sched_text_start+0x8/0x8
[  564.187672]  ? __kthread_parkme+0xcb/0x100
[  564.188528]  ? kthread_blkcg+0x70/0x70
[  564.189333]  ? kthread+0x180/0x1d0
[  564.190052]  ? __issue_discard_cmd+0x480/0x480
[  564.190983]  rewind_stack_do_exit+0x17/0x20

[  564.192190] The buggy address belongs to the page:
[  564.193213] page:ffffea0007cc45c0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
[  564.194856] flags: 0x2ffff0000000000()
[  564.195644] raw: 02ffff0000000000 0000000000000000 dead000000000200 0000000000000000
[  564.197247] raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
[  564.198826] page dumped because: kasan: bad access detected

[  564.200299] Memory state around the buggy address:
[  564.201306]  ffff8801f3117b80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  564.202779]  ffff8801f3117c00: 00 00 00 00 00 00 00 00 00 00 00 f3 f3 f3 f3 f3
[  564.204252] >ffff8801f3117c80: f3 f3 f3 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
[  564.205742]                    ^
[  564.206424]  ffff8801f3117d00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  564.207908]  ffff8801f3117d80: f3 f3 f3 f3 f3 f3 f3 f3 00 00 00 00 00 00 00 00
[  564.209389] ==================================================================
[  564.231795] F2FS-fs (loop0): Mounted with checkpoint version = 2

- Location
https://elixir.bootlin.com/linux/v4.18-rc1/source/fs/f2fs/segment.h#L586
	return div_u64((u64)valid_user_blocks(sbi) * 100,
					sbi->user_block_count);
Missing checks on sbi->user_block_count.
Reported-by: Wen Xu <wen.xu@gatech.edu>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>