Pull up following revision(s) (requested by ozaki-r in ticket #1642):

sys/net/route.c: revision 1.237

route: run workqueue kthreads with KERNEL_LOCK unless NET_MPSAFE

Without KERNEL_LOCK, rt_timer_work and rt_free_work can run in parallel
with other LWPs running in the network stack, which eventually results
in say use-after-free of a deleted route.

(martin)