pf: fix synproxy to local

When we're synproxy-ing a connection that's going to us (as opposed to a
forwarded one) we wound up trying to send out the pf-generated tcp
packets through pf_intr(), which called ip(6)_output(). That doesn't
work all that well for packets that are destined for us, so in that case
we must call ip(6)_input() instead.

MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D31853