bcache: fix cache_set_flush() NULL pointer dereference on OOM

bcache: fix cache_set_flush() NULL pointer dereference on OOM

[ Upstream commit f8b11260a445169989d01df75d35af0f56178f95 ]

When bch_cache_set_alloc() fails to kzalloc the cache_set, the
asyncronous closure handling tries to dereference a cache_set that
hadn't yet been allocated inside of cache_set_flush() which is called
by __cache_set_unregister() during cleanup.  This appears to happen only
during an OOM condition on bcache_register.

Signed-off-by: Eric Wheeler <bcache@linux.ewheeler.net>
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>