MFC r355881:
Leave multicast group before reaping and committing state for both
IPv4 and IPv6.

This fixes a regression issue after r349369. When trying to exit a
multicast group before closing the socket, a multicast leave packet
should be sent.

Differential Revision: https://reviews.freebsd.org/D22848
PR: 242677
Reviewed by: bz (network)
Tested by: Aleksandr Fedorov <aleksandr.fedorov@itglobal.com>
Sponsored by: Mellanox Technologies