ext4: fix corruption when online resizing a fs with 1K block size

Subtracting the number of the first data block places the superblock
backups one block too early, corrupting the file system. When the block
size is larger than 1K, the first data block is 0, so the subtraction
has no effect and no corruption occurs.
Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
CC: stable@vger.kernel.org

fs/ext4/resize.c

@@ -1660,12 +1660,10 @@ static int ext4_group_extend_no_check(struct super_block *sb,
 		err = err2;
 
 	if (!err) {
-		ext4_fsblk_t first_block;
-		first_block = ext4_group_first_block_no(sb, 0);
 		if (test_opt(sb, DEBUG))
 			printk(KERN_DEBUG "EXT4-fs: extended group to %llu "
 			       "blocks\n", ext4_blocks_count(es));
-		update_backups(sb, EXT4_SB(sb)->s_sbh->b_blocknr - first_block,
+		update_backups(sb, EXT4_SB(sb)->s_sbh->b_blocknr,
 			       (char *)es, sizeof(struct ext4_super_block), 0);
 	}
 	return err;