inet: Simplify if_multiaddrs iteration.
Similar to 2cd6ad766eb23 for inet6 drop ifma_restart use, creating more
problems than solving. It is no longer needed after epoch introduction.
While there, add NULL check for ifma_ifp in igmp_change_state(), that
sometimes caused panics on interface destruction.
MFC after: 2 weeks
(cherry picked from commit 1e9482f4331bdce775061bea66ff54a6a79d5245)