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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31889