Revise FIB lookups per second benchmarking routines.

Add a LPS benchmark variant which introduces artificial dependencies
between successive lookups. While here, instead of writing the results
from the lookups to a huge array, add them to an accumulator, in a more
lightweight attempt at preventing the CPU's OOO machinery from
discarding the lookup results if they would be completely unused.

net.route.test.run_lps_rnd measures LPS throughput with independent
uniformly random keys

net.route.test.run_lps_seq measures LPS throughput with uniformly
random keys with artificial interdependencies
Reviewed by: melifaro
MFC after: 7 days
Differential Revision: https://reviews.freebsd.org/D30096