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
Approved by: re(cperciva)

(cherry picked from commit 28a5d88f7091d1fc72f4f1bd8562d3c8b15883f5)
(cherry picked from commit b656161b84dce0322ab00d7958267fa47cda6318)