Need to wait for epoch callbacks to complete before detaching a

Need to wait for epoch callbacks to complete before detaching a
network interface.

This particularly manifests itself when an INP has multicast options
attached during a network interface detach. Then the IPv4 and IPv6
leave group call which results from freeing the multicast address, may
access a freed ifnet structure. These are the steps to reproduce:

service mdnsd onestart # installed from ports

ifconfig epair create
ifconfig epair0a 0/24 up
ifconfig epair0a destroy

Tested by:	pho @
MFC after:	1 week
Sponsored by:	Mellanox Technologies