Rename unp_pcb_lock2().

unp_pcb_lock_pair() seems like a better name. Also make it handle the
case where the two sockets are the same instead of making callers do it.
No functional change intended.

Reviewed by: glebius, kevans, kib
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26296