[XFS] When calculating the number of pages to probe for an unwritten extent,

use the size of the extent, not the page count of the pagebuf which is
initialized to zero.

SGI Modid: 2.5.x-xfs:slinx:159206a

fs/xfs/linux/xfs_aops.c

@@ -461,7 +461,8 @@ map_unwritten(
 		struct page		*page;
 
 		tlast = i_size_read(inode) >> PAGE_CACHE_SHIFT;
-		tloff = min(tlast, start_page->index + pb->pb_page_count - 1);
+		tloff = (mp->pbm_offset + mp->pbm_bsize) >> PAGE_CACHE_SHIFT;
+		tloff = min(tlast, tloff);
 		for (tindex = start_page->index + 1; tindex < tloff; tindex++) {
 			page = probe_unwritten_page(mapping, tindex, mp, pb,
 						PAGE_CACHE_SIZE, &bs, bbits);