cgroup: Skip unified hier controllers (v2)

The unified hierarchy controller doesn't have any name, so criu hangs
while parsing this sort of strings.

Let's skip those for now, until we implement cgroup2 support.

https://github.com/xemul/criu/issues/252

v2: Code comments
Reported-and-tested-by: Adrian Reber <adrian@lisas.de>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/proc_parse.c

@@ -2252,8 +2252,19 @@ int parse_cgroup_file(FILE *f, struct list_head *retl, unsigned int *n)
 		 * 2:name=systemd:/user.slice/user-1000.slice/session-1.scope
 		 */
 		name = strchr(buf, ':');
-		if (name)
+		if (name) {
 			path = strchr(++name, ':');
+			if (*name == ':') {
+				/*
+				 * It's unified hierarchy. On kernels with legacy
+				 * tree this item is added automatically, so we
+				 * can just skip one. For those with full unified
+				 * support is on ... we need to write new code.
+				 */
+				xfree(ncc);
+				continue;
+			}
+		}
 		if (!name || !path) {
 			pr_err("Failed parsing cgroup %s\n", buf);
 			xfree(ncc);
@@ -2417,6 +2428,13 @@ int collect_controllers(struct list_head *cgroups, unsigned int *n_cgroups)
 		}
 		controllers++;
 
+		if (*controllers == ':')
+			/*
+			 * Unified hier. See comment in parse_cgroup_file
+			 * for more details.
+			 */
+			continue;
+
 		off = strchr(controllers, ':');
 		if (!off) {
 			pr_err("Unable to parse \"%s\"\n", buf);