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.
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.
Tested by: Guido van Rooij <email@example.com>
MFC after: 1 week
Sponsored by: Mellanox Technologies