new_sync_write(): discard ->ki_pos unless the return value is positive

That allows ->write_iter() instances much more convenient life wrt
iocb->ki_pos (and fixes several filesystems with borderline POSIX
violations when zero-length write succeeds and changes the current
position).
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/read_write.c

@@ -477,7 +477,8 @@ static ssize_t new_sync_write(struct file *filp, const char __user *buf, size_t
 
 	ret = filp->f_op->write_iter(&kiocb, &iter);
 	BUG_ON(ret == -EIOCBQUEUED);
-	*ppos = kiocb.ki_pos;
+	if (ret > 0)
+		*ppos = kiocb.ki_pos;
 	return ret;
 }