Added syscall to update filestats (size) from pager.

After flushing written/truncated pages, stats are updated
so that vfs is synced with new file info such as its size.

tasks/fs0/main.c

@@ -92,6 +92,11 @@ void handle_fs_requests(void)
 		pager_sys_write(sender, (unsigned long)mr[0], (unsigned long)mr[1],
 			        (unsigned long)mr[2], (void *)mr[3]);
 		break;
+	case L4_IPC_TAG_PAGER_UPDATE_STATS:
+		pager_update_stats(sender, (unsigned long)mr[0],
+				   (unsigned long)mr[1]);
+		break;
+
 	default:
 		printf("%s: Unrecognised ipc tag (%d) "
 		       "received from tid: %d. Ignoring.\n", __TASKNAME__,