[GFS2] Fix page_mkwrite truncation race path

There was a bug in the truncation/invalidation race path for
->page_mkwrite for gfs2. It ought to return 0 so that the effect is the
same as if the page was truncated at any of the other points at which
the page_lock is dropped. This will result in the restart of the whole
page fault path. If it was due to a real truncation (as opposed to an
invalidate because we let a glock go) then the ->fault path will pick
that up when it gets called again.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/ops_file.c

@@ -394,6 +394,7 @@ static int gfs2_page_mkwrite(struct vm_area_struct *vma, struct page *page)
 	last_index = ip->i_inode.i_size >> PAGE_CACHE_SHIFT;
 	if (page->index > last_index)
 		goto out_unlock_page;
+	ret = 0;
 	if (!PageUptodate(page) || page->mapping != ip->i_inode.i_mapping)
 		goto out_unlock_page;
 	if (gfs2_is_stuffed(ip)) {