[PATCH] fix calculation of meta_bg descriptor backups

"group" is the group where the backup will be placed, and is
initialized to zero in the declaration. This meant that backups for
meta_bg descriptors were erroneously written to the backup block group
descriptors in groups 1 and (desc_per_block-1).

Reproduction information:
  mke2fs -Fq -t ext4 -b 1024 -O ^resize_inode /tmp/foo.img 16G
  truncate -s 24G /tmp/foo.img
  losetup /dev/loop0 /tmp/foo.img
  mount /dev/loop0 /mnt
  resize2fs /dev/loop0
  umount /dev/loop0
  dd if=/dev/zero of=/dev/loop0 bs=1024 count=2
  e2fsck -fy /dev/loop0
  losetup -d /dev/loop0
Signed-off-by: Andy Leiserson <andy@leiserson.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

fs/ext4/resize.c

@@ -1039,7 +1039,7 @@ static int reserve_backup_gdb(handle_t *handle, struct inode *inode,
  * do not copy the full number of backups at this time.  The resize
  * which changed s_groups_count will backup again.
  */
-static void update_backups(struct super_block *sb, int blk_off, char *data,
+static void update_backups(struct super_block *sb, sector_t blk_off, char *data,
 			   int size, int meta_bg)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
@@ -1064,7 +1064,7 @@ static void update_backups(struct super_block *sb, int blk_off, char *data,
 		group = ext4_list_backups(sb, &three, &five, &seven);
 		last = sbi->s_groups_count;
 	} else {
-		group = ext4_meta_bg_first_group(sb, group) + 1;
+		group = ext4_get_group_number(sb, blk_off) + 1;
 		last = (ext4_group_t)(group + EXT4_DESC_PER_BLOCK(sb) - 2);
 	}