ext3: fix online resize bug

There is a bug when we are trying to verify that the reserve inode's
double indirect blocks point back to the primary gdt blocks.  The fix is
obvious, we need to mod the gdb count by the addr's per block.  You can
verify this with the following test case

dd if=/dev/zero of=disk1 seek=1024 count=1 bs=100M
losetup /dev/loop1 disk1
pvcreate /dev/loop1
vgcreate loopvg1 /dev/loop1
lvcreate -l 100%VG loopvg1 -n looplv1
mkfs.ext3 -J size=64 -b 1024 /dev/loopvg1/looplv1
mount /dev/loopvg1/looplv1 /mnt/loop
dd if=/dev/zero of=disk2 seek=1024 count=1 bs=50M
losetup /dev/loop2 disk2
pvcreate /dev/loop2
vgextend loopvg1 /dev/loop2
lvextend -l 100%VG /dev/loopvg1/looplv1
resize2fs /dev/loopvg1/looplv1

without this patch the resize2fs fails, with it the resize2fs succeeds.
Signed-off-by: NJosef Bacik <jbacik@redhat.com>
Acked-by: NAndreas Dilger <adilger@sun.com>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/ext3/resize.c

@@ -580,7 +580,8 @@ static int reserve_backup_gdb(handle_t *handle, struct inode *inode,
 	}
 
 	blk = EXT3_SB(sb)->s_sbh->b_blocknr + 1 + EXT3_SB(sb)->s_gdb_count;
-	data = (__le32 *)dind->b_data + EXT3_SB(sb)->s_gdb_count;
+	data = (__le32 *)dind->b_data + (EXT3_SB(sb)->s_gdb_count %
+					 EXT3_ADDR_PER_BLOCK(sb));
 	end = (__le32 *)dind->b_data + EXT3_ADDR_PER_BLOCK(sb);
 
 	/* Get each reserved primary GDT block and verify it holds backups */