Make sure the multicast release tasks are properly drained when
destroying a VNET or a network interface.

Else the inm release tasks, both IPv4 and IPv6 may cause a panic
accessing a freed VNET or network interface.

Reviewed by: jmg@
Discussed with: bz@
Differential Revision: https://reviews.freebsd.org/D24914
MFC after: 1 week
Sponsored by: Mellanox Technologies