ext3: 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).

Jan Kara: Added a comment and changed check to use cpu-endian value.
Signed-off-by: N"Frans van de Wiel" <fvdw@fvdw.eu>
Signed-off-by: NJan Kara <jack@suse.cz>

fs/ext3/balloc.c

@@ -1583,6 +1583,12 @@ ext3_fsblk_t ext3_new_blocks(handle_t *handle, struct inode *inode,
 		if (!gdp)
 			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