netlink: make the maximum allowed netlink socket buffer runtime tunable.

Dumping large routng tables (>1M paths with multipath) require the socket
buffer which is larger than the currently defined limit.
Allow the limit to be set in runtime, similar to kern.ipc.maxsockbuf.

Reported by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
MFC after: 1 day