mem-snap: Redefine CLI options for mem snapshot

They are not documented, thus OK for now. Two options --
* one to specify where the parent images are
* one to reset dirty memory tracking

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mem.c

@@ -45,9 +45,15 @@ static int task_reset_dirty_track(int pid)
 	int fd, ret;
 	char cmd[] = "4";
 
-	if (!opts.mem_snapshot)
+	if (!opts.track_mem)
 		return 0;
 
+	if (!kerndat_has_dirty_track) {
+		pr_err("Kernel doesn't support dirty tracking. "
+				"No snapshot available.\n");
+		return -1;
+	}
+
 	pr_info("Reset %d's dirty tracking\n", pid);
 	fd = open_proc_rw(pid, "clear_refs");
 	if (fd < 0)
@@ -71,15 +77,6 @@ static struct mem_snap_ctx *mem_snap_init(struct parasite_ctl *ctl)
 	int p_fd, pm_fd;
 	PagemapHead *h;
 
-	if (!opts.mem_snapshot)
-		return NULL;
-
-	if (!kerndat_has_dirty_track) {
-		pr_err("Kernel doesn't support dirty tracking. "
-				"No snapshot available.\n");
-		return ERR_PTR(-1);
-	}
-
 	p_fd = get_service_fd(PARENT_FD_OFF);
 	if (p_fd < 0) {
 		pr_debug("Will do full memory dump\n");