ext4: fix a race which could leak memory in ext4_groupinfo_create_slab()

In ext4_groupinfo_create_slab, we create ext4_groupinfo_caches within
ext4_grpinfo_slab_create_mutex, but set it outside the lock, and there
does exist some case that we may create it twice and causes a memory
leak.  So set it before we call mutex_unlock.
Signed-off-by: NTao Ma <boyu.mt@taobao.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -2404,14 +2404,14 @@ static int ext4_groupinfo_create_slab(size_t size)
 					slab_size, 0, SLAB_RECLAIM_ACCOUNT,
 					NULL);
 
+	ext4_groupinfo_caches[cache_index] = cachep;
+
 	mutex_unlock(&ext4_grpinfo_slab_create_mutex);
 	if (!cachep) {
 		printk(KERN_EMERG "EXT4: no memory for groupinfo slab cache\n");
 		return -ENOMEM;
 	}
 
-	ext4_groupinfo_caches[cache_index] = cachep;
-
 	return 0;
 }