ext4: Fix lockdep recursive locking warning

In ext4_mb_init_group(), if the filesystem block size is less than
PAGE_SIZE/2, the code tries to grab alloc_sem for multiple block
groups in a loop.  We need to allow for this by using
down_write_nested() and passing in the loop index as a lock subclass
number.  This works because no other code path needs to take multiple
alloc_sem's.  Note that lockdep will fail for filesystem blocksize
smaller than to PAGE_SIZE/16k.  (e.g., a 1k filesystem blocksize with
a 32k page size, or a 2k filesystem blocksize with a 64k blocksize,
etc.)
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -1780,7 +1780,7 @@ int ext4_mb_get_buddy_cache_lock(struct super_block *sb, ext4_group_t group)
 		 * no block allocation going on in any
 		 * of that groups
 		 */
-		down_write(&grp->alloc_sem);
+		down_write_nested(&grp->alloc_sem, i);
 	}
 	return i;
 }