mbcache: don't BUG() if entry cache cannot be allocated

mbcache can be a module that is loaded long after startup, when someone
asks to mount an ext2 or ext4 filesystem.  Therefore it should not BUG()
if kmem_cache_create() fails, but rather just fail the module load.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/mbcache.c

@@ -420,7 +420,8 @@ static int __init mbcache_init(void)
 	mb_entry_cache = kmem_cache_create("mbcache",
 				sizeof(struct mb_cache_entry), 0,
 				SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
-	BUG_ON(!mb_entry_cache);
+	if (!mb_entry_cache)
+		return -ENOMEM;
 	return 0;
 }