netfs: Fix READ/WRITE confusion when calling iov_iter_xarray()

Fix netfs_clear_unread() to pass READ to iov_iter_xarray() instead of WRITE
(the flag is about the operation accessing the buffer, not what sort of
access it is doing to the buffer).

Fixes: 3d3c950 ("netfs: Provide readahead and readpage netfs helpers")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-afs@lists.infradead.org
cc: ceph-devel@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: v9fs-developer@lists.sourceforge.net
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/162729351325.813557.9242842205308443901.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/162886603464.3940407.3790841170414793899.stgit@warthog.procyon.org.uk
Link: https://lore.kernel.org/r/163239074602.1243337.14154704004485867017.stgit@warthog.procyon.org.uk

fs/netfs/read_helper.c

@@ -150,7 +150,7 @@ static void netfs_clear_unread(struct netfs_read_subrequest *subreq)
 {
 	struct iov_iter iter;
 
-	iov_iter_xarray(&iter, WRITE, &subreq->rreq->mapping->i_pages,
+	iov_iter_xarray(&iter, READ, &subreq->rreq->mapping->i_pages,
 			subreq->start + subreq->transferred,
 			subreq->len   - subreq->transferred);
 	iov_iter_zero(iov_iter_count(&iter), &iter);