hpts/lro: make tcp_lro_flush_tcphpts() and tcp_run_hpts() pointers

Rename tcp_run_hpts() to tcp_hpts_softlock() to better describe its
function. This makes loadable hpts.ko working correctly with LRO.

Reviewed by: tuexen, rrs
Differential Revision: https://reviews.freebsd.org/D42858