bcache: fix cache_set_flush() NULL pointer dereference on OOM

commit f8b11260 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>

drivers/md/bcache/super.c

@@ -1295,6 +1295,9 @@ static void cache_set_flush(struct closure *cl)
 	set_bit(CACHE_SET_STOPPING_2, &c->flags);
 	wake_up(&c->alloc_wait);
 
+	if (!c)
+		closure_return(cl);
+
 	bch_cache_accounting_destroy(&c->accounting);
 
 	kobject_put(&c->internal);