ifnet: Remove if_getamcount()

All uses of this function were incorrect. if_amcount is a reference
count which tracks the number of times the network stack internally set
IFF_ALLMULTI. (if_pcount is the corresponding counter for IFF_PROMISC.)

Remove if_getamcount() and fix up callers to get the number of assigned
multicast addresses instead, since that's what they actually want.

Sponsored by: Klara, Inc.
Reviewed by: zlei, glebius
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D46523

(cherry picked from commit 408c909dc64f77d2696d6fec77a2e0b00255cf96)