fix up iter on short count in fuse_direct_io()

fuse_direct_io() can end up advancing the iterator by more than the amount
of data read or written.  This case is handled by the generic code if going
through ->direct_IO(), but not in the FOPEN_DIRECT_IO case.

Fix by reverting the extra bytes from the iterator in case of error or a
short count.

To test: install lxcfs, then the following testcase
  int fd = open("/var/lib/lxcfs/proc/uptime", O_RDONLY);
  sendfile(1, fd, NULL, 16777216);
  sendfile(1, fd, NULL, 16777216);
will spew WARN_ON() in iov_iter_pipe().
Reported-by: Peter Geis <pgwipeout@gmail.com>
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Fixes: 3c3db095 ("fuse: use iov_iter based generic splice helpers")
Cc: <stable@vger.kernel.org> # v5.1
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -1465,6 +1465,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 		}
 		ia = NULL;
 		if (nres < 0) {
+			iov_iter_revert(iter, nbytes);
 			err = nres;
 			break;
 		}
@@ -1473,8 +1474,10 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 		count -= nres;
 		res += nres;
 		pos += nres;
-		if (nres != nbytes)
+		if (nres != nbytes) {
+			iov_iter_revert(iter, nbytes - nres);
 			break;
+		}
 		if (count) {
 			max_pages = iov_iter_npages(iter, fc->max_pages);
 			ia = fuse_io_alloc(io, max_pages);