runc delete: fix for rootless cgroup + ro cgroupfs

An issue with runc 1.2.0 was reported to buildkit, in which
runc delete returns with an error, with the log saying:

> unable to destroy container: unable to remove container's cgroup: open /sys/fs/cgroup/snschvixiy3s74w74fjantrdg: no such file or directory

Apparently, what happens is runc is running with no cgroup access
(because /sys/fs/cgroup is mounted read-only). In this case error to
create a cgroup path (in runc create/run) is ignored, but cgroup removal
(in runc delete) is not.

This is caused by commit d3d7f7d, which changes the cgroup removal
logic in RemovePath. In the current code, if the initial rmdir has
failed (in this case with EROFS), but the subsequent os.ReadDir returns
ENOENT, it is returned (instead of being ignored -- as the path does not
exist and so there is nothing to remove).

Here is the minimal fix for the issue.

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

libcontainer/cgroups/utils.go

@@ -257,7 +257,10 @@ func RemovePath(path string) error {
 	}
 
 	infos, err := os.ReadDir(path)
-	if err != nil && !os.IsNotExist(err) {
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
 		return err
 	}
 	for _, info := range infos {