bcache: unregister reboot notifier if bcache fails to unregister device

In bcache_init() function it forgot to unregister reboot notifier if
bcache fails to unregister a block device.  This commit fixes this.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Tested-by: Joshua Schmid <jschmid@suse.com>
Tested-by: Eric Wheeler <bcache@linux.ewheeler.net>
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@fb.com>