tcp: cleanup functions related to socket option handling

Consistently only pass the inp and the sopt around. Don't pass the
so around, since in a upcoming commit tcp_ctloutput_set() will be
called from a context different from setsockopt(). Also expect
the inp to be locked when calling tcp_ctloutput_[gs]et(), this is
also required for the upcoming use by tcpsso, a command line tool
to set socket options.
Reviewed by: glebius, rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D34151