Kernel crashed when register a duplicate cache device, the call trace is
bellow:
[  417.643790] CPU: 1 PID: 16886 Comm: bcache-register Tainted: G
   W  OE    4.15.5-amd64-preempt-sysrq-20171018 #2
[  417.643861] Hardware name: LENOVO 20ERCTO1WW/20ERCTO1WW, BIOS
N1DET41W (1.15 ) 12/31/2015
[  417.643870] RIP: 0010:bdevname+0x13/0x1e
[  417.643876] RSP: 0018:ffffa3aa9138fd38 EFLAGS: 00010282
[  417.643884] RAX: 0000000000000000 RBX: ffff8c8f2f2f8000 RCX: ffffd6701f8
c7edf
[  417.643890] RDX: ffffa3aa9138fd88 RSI: ffffa3aa9138fd88 RDI: 00000000000
00000
[  417.643895] RBP: ffffa3aa9138fde0 R08: ffffa3aa9138fae8 R09: 00000000000
1850e
[  417.643901] R10: ffff8c8eed34b271 R11: ffff8c8eed34b250 R12: 00000000000
00000
[  417.643906] R13: ffffd6701f78f940 R14: ffff8c8f38f80000 R15: ffff8c8ea7d
90000
[  417.643913] FS:  00007fde7e66f500(0000) GS:ffff8c8f61440000(0000) knlGS:
0000000000000000
[  417.643919] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  417.643925] CR2: 0000000000000314 CR3: 00000007e6fa0001 CR4: 00000000003
606e0
[  417.643931] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 00000000000
00000
[  417.643938] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 00000000000
00400
[  417.643946] Call Trace:
[  417.643978]  register_bcache+0x1117/0x1270 [bcache]
[  417.643994]  ? slab_pre_alloc_hook+0x15/0x3c
[  417.644001]  ? slab_post_alloc_hook.isra.44+0xa/0x1a
[  417.644013]  ? kernfs_fop_write+0xf6/0x138
[  417.644020]  kernfs_fop_write+0xf6/0x138
[  417.644031]  __vfs_write+0x31/0xcc
[  417.644043]  ? current_kernel_time64+0x10/0x36
[  417.644115]  ? __audit_syscall_entry+0xbf/0xe3
[  417.644124]  vfs_write+0xa5/0xe2
[  417.644133]  SyS_write+0x5c/0x9f
[  417.644144]  do_syscall_64+0x72/0x81
[  417.644161]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[  417.644169] RIP: 0033:0x7fde7e1c1974
[  417.644175] RSP: 002b:00007fff13009a38 EFLAGS: 00000246 ORIG_RAX: 0000000
000000001
[  417.644183] RAX: ffffffffffffffda RBX: 0000000001658280 RCX: 00007fde7e1c
1974
[  417.644188] RDX: 000000000000000a RSI: 0000000001658280 RDI: 000000000000
0001
[  417.644193] RBP: 000000000000000a R08: 0000000000000003 R09: 000000000000
0077
[  417.644198] R10: 000000000000089e R11: 0000000000000246 R12: 000000000000
0001
[  417.644203] R13: 000000000000000a R14: 7fffffffffffffff R15: 000000000000
0000
[  417.644213] Code: c7 c2 83 6f ee 98 be 20 00 00 00 48 89 df e8 6c 27 3b 0
0 48 89 d8 5b c3 0f 1f 44 00 00 48 8b 47 70 48 89 f2 48 8b bf 80 00 00 00 <8
b> b0 14 03 00 00 e9 73 ff ff ff 0f 1f 44 00 00 48 8b 47 40 39
[  417.644302] RIP: bdevname+0x13/0x1e RSP: ffffa3aa9138fd38
[  417.644306] CR2: 0000000000000314

When registering duplicate cache device in register_cache(), after failure
on calling register_cache_set(), bch_cache_release() will be called, then
bdev will be freed, so bdevname(bdev, name) caused kernel crash.

Since bch_cache_release() will free bdev, so in this patch we make sure
bdev being freed if register_cache() fail, and do not free bdev again in
register_bcache() when register_cache() fail.
Signed-off-by: Tang Junhui <tang.junhui@zte.com.cn>
Reported-by: Marc MERLIN <marc@merlins.org>
Tested-by: Michael Lyle <mlyle@lyle.org>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>