ocfs2: Only bug out when page size is larger than cluster size.

In CoW, we have to make sure that the page is already written
out to the disk. So we have a BUG_ON(PageDirty(page)).

In ppc platform we have pagesize=64K, so if the cs=4K, if the
file have fragmented clusters, we will map the page many times.
See this file as an example.
Tree Depth: 0   Count: 19   Next Free Rec: 14
	## Offset        Clusters       Block#          Flags
	0  0             4              2164864         0x2 Refcounted
	1  4             2              9302792         0x2 Refcounted
...

We have to replace the extent recs one by one, so the page with index 0
will be mapped and dirtied twice.

I'd like to leave the BUG_ON there while adding a check so that in
case we meet with an error in other platforms, we can find it easily.
Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

fs/ocfs2/refcounttree.c

@@ -2957,8 +2957,12 @@ static int ocfs2_duplicate_clusters_by_page(handle_t *handle,
 
 		page = grab_cache_page(mapping, page_index);
 
-		/* This page can't be dirtied before we CoW it out. */
-		BUG_ON(PageDirty(page));
+		/*
+		 * In case PAGE_CACHE_SIZE <= CLUSTER_SIZE, This page
+		 * can't be dirtied before we CoW it out.
+		 */
+		if (PAGE_CACHE_SIZE <= OCFS2_SB(sb)->s_clustersize)
+			BUG_ON(PageDirty(page));
 
 		if (!PageUptodate(page)) {
 			ret = block_read_full_page(page, ocfs2_get_block);