[bug] Return a spec on reconnect

[angrycub]

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

[shoenig]

LGTM! Be sure to add a CL entry

[tgross]

LGTM!

From #9757:

safely handle

🤦 Guess not! What's insidious about this bug is that there's a "resources may have leaked" error just after the nil check in the PostRun, where we could have potentially caught this behavior.

[github-actions]

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