tcp: enter network epoch when calling tfb_tcp_fb_fini

We need to enter the network epoch when calling into
tfb_tcp_fb_fini. I noticed this when I hit an assert
running the latest rack

Differential Revision: https://reviews.freebsd.org/D30407
Reviewed by: rrs, tuexen
Sponsored by: Netflix

(cherry picked from commit 086a35562f47917a516d30acc8b78a4884e31a4f)