mnt: Collect mntns root from criu, in case we don't dump mount namespace

In case criu and dumpee live in the same mount namespace there's no
need in getting ns' root from init task. We can get it from criu and
(!) void the root == "/" check, required for namespace case.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -1225,6 +1225,23 @@ int mntns_collect_root(pid_t pid)
 	int ret;
 	char path[PATH_MAX + 1];
 
+	if (!(current_ns_mask & CLONE_NEWNS)) {
+		/*
+		 * If criu and tasks we dump live in the same mount
+		 * namespace, we can just open the root directory.
+		 * All paths resolution would occur relative to criu's
+		 * root. Even if it is not namespace's root, provided
+		 * file paths are resolved, we'd get consistent dump.
+		 */
+		fd = open("/", O_RDONLY | O_DIRECTORY);
+		if (fd < 0) {
+			pr_perror("Can't open root\n");
+			return -1;
+		}
+
+		goto set_root;
+	}
+
 	/*
 	 * If /proc/pid/root links on '/', it signs that a root of the task
 	 * and a root of mntns is the same.
@@ -1252,8 +1269,8 @@ int mntns_collect_root(pid_t pid)
 		return -1;
 	}
 
+set_root:
 	mntns_root = fd;
-
 	return 0;
 }