libct/cg: stop using utils.ProcThreadSelf

We were using utils.ProcThreadSelf since commit 8e8b136c,
which provides two things:
1. locking the OS tread;
2. fallback to /proc/self/task/$TID when /proc/thread-self
   is not available (kernel < 3.17).

Now, (1) is not needed since we only call readlink and not perform any
file data operation, and (2) is not needed here as this code is
only running when openat2 syscall is available, meaning kernel >= v5.6.

Also, check the error from readlink, so when it fails, we do not try to
enhance the error message.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/file.go

@@ -171,10 +171,8 @@ func openFile(dir, file string, flags int) (*os.File, error) {
 		//
 		// TODO: if such usage will ever be common, amend this
 		// to reopen cgroupRootHandle and retry openat2.
-		fdPath, closer := utils.ProcThreadSelf("fd/" + strconv.Itoa(int(cgroupRootHandle.Fd())))
-		defer closer()
-		fdDest, _ := os.Readlink(fdPath)
-		if fdDest != cgroupfsDir {
+		fdDest, fdErr := os.Readlink("/proc/thread-self/fd/" + strconv.Itoa(int(cgroupRootHandle.Fd())))
+		if fdErr == nil && fdDest != cgroupfsDir {
 			// Wrap the error so it is clear that cgroupRootHandle
 			// is opened to an unexpected/wrong directory.
 			err = fmt.Errorf("cgroupRootHandle %d unexpectedly opened to %s != %s: %w",