We can't just use scsi_cd() to get the scsi_cd structure, we have
to grab a live reference to the device. For both callbacks, we're
not inside an open where we already hold a reference to the device.

This fixes device removal/addition under concurrent device access,
which otherwise could result in the below oops.

NULL pointer dereference at 0000000000000010
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP
Modules linked in:
sr 12:0:0:0: [sr2] scsi-1 drive
 scsi_debug crc_t10dif crct10dif_generic crct10dif_common nvme nvme_core sb_edac xl
sr 12:0:0:0: Attached scsi CD-ROM sr2
 sr_mod cdrom btrfs xor zstd_decompress zstd_compress xxhash lzo_compress zlib_defc
sr 12:0:0:0: Attached scsi generic sg7 type 5
 igb ahci libahci i2c_algo_bit libata dca [last unloaded: crc_t10dif]
CPU: 43 PID: 4629 Comm: systemd-udevd Not tainted 4.16.0+ #650
Hardware name: Dell Inc. PowerEdge T630/0NT78X, BIOS 2.3.4 11/09/2016
RIP: 0010:sr_block_revalidate_disk+0x23/0x190 [sr_mod]
RSP: 0018:ffff883ff357bb58 EFLAGS: 00010292
RAX: ffffffffa00b07d0 RBX: ffff883ff3058000 RCX: ffff883ff357bb66
RDX: 0000000000000003 RSI: 0000000000007530 RDI: ffff881fea631000
RBP: 0000000000000000 R08: ffff881fe4d38400 R09: 0000000000000000
R10: 0000000000000000 R11: 00000000000001b6 R12: 000000000800005d
R13: 000000000800005d R14: ffff883ffd9b3790 R15: 0000000000000000
FS:  00007f7dc8e6d8c0(0000) GS:ffff883fff340000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000010 CR3: 0000003ffda98005 CR4: 00000000003606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 ? __invalidate_device+0x48/0x60
 check_disk_change+0x4c/0x60
 sr_block_open+0x16/0xd0 [sr_mod]
 __blkdev_get+0xb9/0x450
 ? iget5_locked+0x1c0/0x1e0
 blkdev_get+0x11e/0x320
 ? bdget+0x11d/0x150
 ? _raw_spin_unlock+0xa/0x20
 ? bd_acquire+0xc0/0xc0
 do_dentry_open+0x1b0/0x320
 ? inode_permission+0x24/0xc0
 path_openat+0x4e6/0x1420
 ? cpumask_any_but+0x1f/0x40
 ? flush_tlb_mm_range+0xa0/0x120
 do_filp_open+0x8c/0xf0
 ? __seccomp_filter+0x28/0x230
 ? _raw_spin_unlock+0xa/0x20
 ? __handle_mm_fault+0x7d6/0x9b0
 ? list_lru_add+0xa8/0xc0
 ? _raw_spin_unlock+0xa/0x20
 ? __alloc_fd+0xaf/0x160
 ? do_sys_open+0x1a6/0x230
 do_sys_open+0x1a6/0x230
 do_syscall_64+0x5a/0x100
 entry_SYSCALL_64_after_hwframe+0x3d/0xa2
Reviewed-by: Lee Duncan <lduncan@suse.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>