ng pppoe(4): Add the required NET_EPOCH section to the hook
disconnection function.

Disconnecting hooks are called outside of NET_EPOCH, but
ng_pppoe_disconnect() calls NG_SEND_DATA_ONLY() which should be called
in NET_EPOCH.

PR: 257067
Reported by: niels=freebsd@bakker.net
Reviewed by: vmaffione (mentor), glebius, donner
Approved by: vmaffione (mentor), glebius, donner
Sponsored by: vstack.com
Differential Revision: https://reviews.freebsd.org/D34185