vrrp: sign the auth_hmac trailer once per advertisement

Signing in vrrp_send_pkt() gave each unicast peer copy of the same
logical advertisement its own sequence and HMAC. The copies now
replicate one authenticated advertisement, identical apart from the
destination and the checksum it feeds, and signing costs one HMAC per
advertisement instead of one per peer.

Signed-off-by: Alexandre Cassen <acassen@gmail.com>
Pointed-out-by: Aditya Dogra <addogra@cisco.com>