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

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