MFC r364072, r364073 and r364102:
Use proper prototype for SYSINIT() functions.
Mark the unused argument using the __unused macro.

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.

Use a static initializer for the multicast free tasks.
This makes the SYSINIT() function updated in r364072 superfluous.

Differential Revision: https://reviews.freebsd.org/D24914
Sponsored by: Mellanox Technologies