ifnet: Remove if_getamcount()

[markj]

Summary

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.

[zlei]

Good catch!