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)