Backport of [bug] Return a spec on reconnect into release/1.3.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #15214 to be assessed for backporting due to the inclusion of the label backport/1.3.x.

The below text is copied from the body of the original PR.

---

This PR changes the behavior on reconnect from the behavior introduced in #9757.

nomad/client/allocrunner/network_manager_linux.go

Lines 100 to 104 in 83b31ea

	if e, ok := err.(*os.PathError); ok && e.Err == syscall.EPERM {
	nsPath := path.Join(nsutil.NetNSRunDir, allocID)
	_, err := os.Stat(nsPath)
	if err == nil {
	return nil, false, nil

If the client enters this case and is returned a nil spec. When the network_hook's Postrun func is called to clean things up, the nil spec causes a fast exit.

nomad/client/allocrunner/network_hook.go

Lines 115 to 118 in 83b31ea

	func (h *networkHook) Postrun() error {
	if h.spec == nil {
	return nil
	}

This PR changes the behavior when we encounter the existing network namespace to return the spec so that the Postrun behavior will run properly to reap the namespace.

Fixes #11096

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