Revert "Merge pull request #2280 from kolyshkin/errors-unwrap"

Using errors.Unwrap() is not the best thing to do, since it returns
nil in case of an error which was not wrapped. More to say,
errors package provides more elegant ways to check for underlying
errors, such as errors.As() and errors.Is().

This reverts commit f8e138855d, reversing
changes made to 6ca9d8e6da.

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

libcontainer/cgroups/utils.go

@@ -576,7 +576,7 @@ func WriteCgroupProc(dir string, pid int) error {
 
 		// EINVAL might mean that the task being added to cgroup.procs is in state
 		// TASK_NEW. We should attempt to do so again.
-		if errors.Unwrap(err) == unix.EINVAL {
+		if isEINVAL(err) {
 			time.Sleep(30 * time.Millisecond)
 			continue
 		}
@@ -586,6 +586,15 @@ func WriteCgroupProc(dir string, pid int) error {
 	return err
 }
 
+func isEINVAL(err error) bool {
+	switch err := err.(type) {
+	case *os.PathError:
+		return err.Err == unix.EINVAL
+	default:
+		return false
+	}
+}
+
 // Since the OCI spec is designed for cgroup v1, in some cases
 // there is need to convert from the cgroup v1 configuration to cgroup v2
 // the formula for BlkIOWeight is y = (1 + (x - 10) * 9999 / 990)