cgv1: do not disable cpuset manager if reserved interface already exists (#16467)

* cgv1: do not disable cpuset manager if reserved interface already exists

This PR fixes a bug where restarting a Nomad Client on a machine using cgroups
v1 (e.g. Ubuntu 20.04) would cause the cpuset cgroups manager to disable itself.

This is being caused by incorrectly interpreting a "file exists" error as
problematic when ensuring the reserved cpuset exists. If we get a "file exists"
error, that just means the Client was likely restarted.

Note that a machine reboot would fix the issue - the groups interfaces are
ephemoral.

* cl: add cl

.changelog/16467.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+client: Fixed a bug where cpuset initialization fails after Client restart
+```

client/lib/cgutil/cpuset_manager_v1.go

@@ -4,6 +4,7 @@ package cgutil
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -59,9 +60,11 @@ func NewCpusetManagerV1(cgroupParent string, _ []uint16, logger hclog.Logger) Cp
 
 	// ensure the reserved cpuset exists, but only copy the mems from the parent if creating the cgroup
 	if err = os.Mkdir(filepath.Join(cgroupParentPath, ReservedCpusetCgroupName), 0755); err != nil {
+		if !errors.Is(err, os.ErrExist) {
 			logger.Warn("failed to ensure reserved cpuset.cpus interface exists; disable cpuset management", "error", err)
 			return new(NoopCpusetManager)
 		}
+	}
 
 	if err = cgroups.WriteFile(filepath.Join(cgroupParentPath, ReservedCpusetCgroupName), "cpuset.mems", parentMems); err != nil {
 		logger.Warn("failed to ensure reserved cpuset.mems interface exists; disable cpuset management", "error", err)