xfs: do not use unchecked extent indices in xfs_bmapi

Make sure to only call xfs_iext_get_ext after we've validate the
extent index when moving on to the next index in xfs_bmapi.

Based on an earlier patch from Lachlan McIlroy.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Lachlan McIlroy <lmcilroy@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/xfs_bmap.c

@@ -4825,12 +4825,13 @@ xfs_bmapi(
 		/*
 		 * Else go on to the next record.
 		 */
-		ep = xfs_iext_get_ext(ifp, ++lastx);
 		prev = got;
-		if (lastx >= nextents)
-			eof = 1;
-		else
+		if (++lastx < nextents) {
+			ep = xfs_iext_get_ext(ifp, lastx);
 			xfs_bmbt_get_all(ep, &got);
+		} else {
+			eof = 1;
+		}
 	}
 	*nmap = n;
 	/*