kernel - Fix several IPV6 issues

kernel - Fix several IPV6 issues

* Replace old critical sections with a lock around global structures,
  primarily the default route list for IPV6.  This fixes numerous MP
  races.

* Use rtrequest*_global() instead of rtrequest*() and remove user route
  socket notification (the rtrequest*_global() code should do the
  notification).  This fixes several issues with the per-cpu route table
  getting out of sync and triggering panics, and also appears to fix
  several connectivity issues.

Reported-by: Francois Tigeot <ftigeot@wolfpond.org>