pf: use sctp_calculate_cksum()

This function is always available, even if the SCTP or SCTP_SUPPORT options
are not set.
That lets us remove an ifdef, and also means we improve pf's SCTP handling
when the options are not set.

MFC after: 3 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D40911

(cherry picked from commit 61e22e9b2905f405170c249a2239f324229b60d1)