Use refcount from "in_joingroup_locked()" when joining multicast
groups. Do not acquire additional references. This makes the IPv4 IGMP
code in line with the IPv6 MLD code.

Background:
The IPv4 multicast code puts an extra reference on the in_multi struct
when joining groups. This becomes visible when using daemons like
igmpproxy from ports, that multicast entries do not disappear from the
output of ifmcstat(8) when multicast streams are disconnected.

This fixes a regression issue after r349762.

While at it factor the ip_mfilter_insert() and ip6_mfilter_insert() calls
to avoid repeated "is_new" check.

Differential Revision: https://reviews.freebsd.org/D22595
Tested by: Guido van Rooij <guido@gvr.org>
Reviewed by: rgrimes (network)
MFC after: 1 week
Sponsored by: Mellanox Technologies