igmp: do not upgrade IGMP version beyond net.inet.igmp.default_version

IGMP requires hosts to use the lowest version they've seen on the
network. When the IGMP timers expire we take the opportunity to upgrade again.
However, we did not take the net.inet.igmp.default_version sysctl
setting into account, so we could end up switching to IGMPv3 even if the
user had requested IGMPv2 or IGMPv1 via the sysctl.

Check V_igmp_default_version before we upgrade the IGMP version.

Reviewed by: adrian
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D41628