ext4: return error on syncfs after shutdown

This is the logic behavior and one that we would like to verify
using a generic fstest similar to xfs/546.

Link: https://lore.kernel.org/fstests/20240830152648.GE6216@frogsfrogsfrogs/
Suggested-by: Darrick J. Wong <[email protected]>
Signed-off-by: Amir Goldstein <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

Author: amir73il

fs/ext4/super.c

@@ -6304,7 +6304,7 @@ static int ext4_sync_fs(struct super_block *sb, int wait)
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 
 	if (unlikely(ext4_forced_shutdown(sb)))
-		return 0;
+		return -EIO;
 
 	trace_ext4_sync_fs(sb, wait);
 	flush_workqueue(sbi->rsv_conversion_wq);