vrrp: reserve the auth_hmac trailer in the advert MTU budget

The VIP fit check bounded the advertised address count by the link MTU
but ignored the authenticated trailer appended after the VIPs. A full
VIP list plus the trailer could overshoot the MTU and fragment the
advert. Reserve the trailer length before deriving the count.

Signed-off-by: Alexandre Cassen <acassen@gmail.com>