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