fix ITER_PIPE interaction with direct_IO

by making sure we call iov_iter_advance() on original
iov_iter even if direct_IO (done on its copy) has returned 0.
It's a no-op for old iov_iter flavours and does the right thing
(== truncation of the stuff we'd allocated, but not filled) in
ITER_PIPE case.  Failures (e.g. -EIO) get caught and dealt with
by cleanup in generic_file_read_iter().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/xfs/xfs_file.c

@@ -317,7 +317,7 @@ xfs_file_dio_aio_read(
 	data = *to;
 	ret = __blockdev_direct_IO(iocb, inode, target->bt_bdev, &data,
 			xfs_get_blocks_direct, NULL, NULL, 0);
-	if (ret > 0) {
+	if (ret >= 0) {
 		iocb->ki_pos += ret;
 		iov_iter_advance(to, ret);
 	}