netinet6: require network epoch for in6_pcbconnect()

This removes recursive epoch entry in the syncache case. Fixes
unprotected access to V_in6_ifaddrhead in in6_pcbladdr(), as
well as access to prison IP address lists. It also matches what
IPv4 in_pcbconnect() does.

Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D38355