Add fib lookup testing module.

This module intended to measure performance of routing lookups.

Uses a list of IP addresses specified by sysctl one-by-one.
Performance testing is triggered by changing sysctl OID with a number of lookups to execute.
Lookups are done by the chunks of 10K routes, entering/exiting epoch on
chunk granularity to amortise cost.

Example:
make -C sys/modules/test/fib_lookup unload load
for i in cat ~/ip4.txt; do sysctl net.route.test.add_inet_addr=$i; done
for i in cat ~/ip6.txt; do sysctl net.route.test.add_inet6_addr=$i; done

sysctl net.route.test.run_inet=10000000

dmesg | tail

Dec 13 23:24:05 current kernel: 10000000 packets in 417240173 nanoseconds, 23967011 pps
Dec 13 23:24:06 current kernel: run: 10000000 packets vnet 0xfffff80003073f00
Dec 13 23:24:07 current kernel: 10000000 packets in 423086254 nanoseconds, 23635842 pps

Differential Revision: https://reviews.freebsd.org/D27604