net: Enter a net epoch around protocol if_up/down notifications

When traversing a list of interface addresses, we need to be in a net
epoch section, and protocol ctlinput routines need a stable reference to
the address.

Reported by: syzbot+3219af764ead146a3a4e@syzkaller.appspotmail.com
Reviewed by: kp, melifaro
Sponsored by: The FreeBSD Foundation

(cherry picked from commit b1e6a792d68e9c59740d5e925405d8d4343d099b)