gfs2: use iomap_readpage for blocksize == PAGE_SIZE

We only use iomap_readpage for pages that don't have buffer heads
attached yet: iomap_readpage would otherwise read pages from disk that
are marked buffer_uptodate() but not PageUptodate().  Those pages may
actually contain data more recent than what's on disk.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>

fs/gfs2/aops.c

@@ -508,9 +508,13 @@ static int __gfs2_readpage(void *file, struct page *page)
 {
 	struct gfs2_inode *ip = GFS2_I(page->mapping->host);
 	struct gfs2_sbd *sdp = GFS2_SB(page->mapping->host);
+
 	int error;
 
-	if (gfs2_is_stuffed(ip)) {
+	if (i_blocksize(page->mapping->host) == PAGE_SIZE &&
+	    !page_has_buffers(page)) {
+		error = iomap_readpage(page, &gfs2_iomap_ops);
+	} else if (gfs2_is_stuffed(ip)) {
 		error = stuffed_readpage(ip, page);
 		unlock_page(page);
 	} else {