igmp: apply net.inet.igmp.default_version to existing interfaces

IGMP requires hosts to use the lowest version they have seen on the network.
When net.inet.igmp.default_version is changed, we do not change IGMP
version used by interface even if the interface use an higher version than
V_igmp_default_version. So we could send IGMPv3 even if the user has requested
IGMPv2 or IGMPv1 via the sysctl.

Change IGMP version for each interface when V_igmp_default_version is smaller
than the version used by the interface.

Pull Request: https://github.com/freebsd/freebsd-src/pull/1615
Differential Revision: https://reviews.freebsd.org/D50071