pf: Use a macro to get the hash row in pf_find_state_byid()

This seems a bit preferable to open-coding it. No functional change
intended.

Reviewed by: kp, glebius
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D49518

(cherry picked from commit 27f70efebf1d9424462f291e9d04e62272083aa7)