Lock down parallel router_info list for tracking multicast IGMP

Lock down parallel router_info list for tracking multicast IGMP
versions of various routers seen:

- Introduce igmp_mtx.
- Protect global variable 'router_info_head' and list fields
  in struct router_info with this mutex, as well as
  igmp_timers_are_running.
- find_rti() asserts that the caller acquires igmp_mtx.
- Annotate a failure to check the return value of
  MALLOC(..., M_NOWAIT).