vfs: make sync_filesystem return errors from ->sync_fs

Strangely, sync_filesystem ignores the return code from the ->sync_fs
call, which means that syscalls like syncfs(2) never see the error.
This doesn't seem right, so fix that.

Signed-off-by: Darrick J. Wong <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>

fs/sync.c

···
  */
 int sync_filesystem(struct super_block *sb)
 {
-	int ret;
+	int ret = 0;
 
 	/*
 	 * We need to be protected against the filesystem going from
···
 	 * at a time.
 	 */
 	writeback_inodes_sb(sb, WB_REASON_SYNC);
-	if (sb->s_op->sync_fs)
-		sb->s_op->sync_fs(sb, 0);
+	if (sb->s_op->sync_fs) {
+		ret = sb->s_op->sync_fs(sb, 0);
+		if (ret)
+			return ret;
+	}
 	ret = sync_blockdev_nowait(sb->s_bdev);
-	if (ret < 0)
+	if (ret)
 		return ret;
 
 	sync_inodes_sb(sb);
-	if (sb->s_op->sync_fs)
-		sb->s_op->sync_fs(sb, 1);
+	if (sb->s_op->sync_fs) {
+		ret = sb->s_op->sync_fs(sb, 1);
+		if (ret)
+			return ret;
+	}
 	return sync_blockdev(sb->s_bdev);
 }
 EXPORT_SYMBOL(sync_filesystem);