ext2: Avoid loading bitmaps for full groups during block allocation

There is no point in loading bitmap for groups which are completely full.
This causes noticeable performance problems (and memory pressure) on small
systems with large full filesystem
(http://marc.info/?l=linux-ext4&m=126843108314310&w=2).

Port of the same ext3 patch.

Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext2/balloc.c

@@ -1331,6 +1331,12 @@ retry_alloc:
 			goto io_error;
 
 		free_blocks = le16_to_cpu(gdp->bg_free_blocks_count);
+		/*
+		 * skip this group (and avoid loading bitmap) if there
+		 * are no free blocks
+		 */
+		if (!free_blocks)
+			continue;
 		/*
 		 * skip this group if the number of
 		 * free blocks is less than half of the reservation