sctp_calculate_cksum() is defined only when SCTP_WITH_NO_CSUM is not
defined. If it is, just leave the checksum initialized to 0.
That doesn't make sense, since SCTP_WITH_NO_CSUM was introduced ages ago just to test how the performance is with checksum offloading. Not computing the checksum in ipfw doesn't make sense...
I suggest that I will just retire SCTP_WITH_NO_CSUM in the source tree. Would that be acceptable?