[XFS] remove some unbounded loops from the unwritten and unmapped page

processing code. As files get larger, these code paths have the
potential to hog the cpu for long periods of time. Just cap the
unmapped page case, and the unwritten one is supposed to be
stopping at the end of the extent anyway.

SGI Modid: 2.5.x-xfs:slinx:141609a

fs/xfs/linux/xfs_aops.c

@@ -288,6 +288,8 @@ probe_unmapped_cluster(
 	 */
 	if (bh == head) {
 		tlast = inode->i_size >> PAGE_CACHE_SHIFT;
+		/* Prune this back to avoid pathological behavior */
+		tlast = min(tlast, startpage->index + 64);
 		for (tindex = startpage->index + 1; tindex < tlast; tindex++) {
 			len = probe_unmapped_page(mapping, tindex,
 							PAGE_CACHE_SIZE);
@@ -405,6 +407,7 @@ map_unwritten(
 		struct page		*page;
 
 		tlast = inode->i_size >> PAGE_CACHE_SHIFT;
+		tlast = min(tlast, start_page->index + pb->pb_page_count - 1);
 		for (tindex = start_page->index + 1; tindex < tlast; tindex++) {
 			page = probe_unwritten_page(mapping, tindex, mp, pb,
 					PAGE_CACHE_SIZE, &bs);