ipmr: change igmpmsg_netlink_event() prototype

igmpmsg_netlink_event() first argument can be marked const.

igmpmsg_netlink_event() reads mrt->net and mrt->id,
both being set once in mr_table_alloc().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>