sockets/tcp: quick fix for regression with SO_REUSEPORT_LB

There was a long living problem that pr_listen is called every time on
consecutive listen(2) syscalls. Up until today it produces spurious TCP
state change events in tracing software and other harmless problems. But
with 7cbb6b6e28db we started to call LIST_REMOVE() twice on the same
entry.

This is quite ugly, but quick and robust fix against regression, that we
decided to put in the scope of the January stabilization week. A better
refactoring will happen later.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D48703
Fixes: 7cbb6b6e28db33095a1cf7a8887921a5ec969824