Let the RACK stack take IP options into account when computing the space for user data in TCP segments

[tuexen]

Summary

When the RACK stack computes the space for user data in a TCP segment, it wasn't taking the IP level options into account. This patch fixes this. It also corrects a KASSERT and adds the protection code missing also from the default stack and to be added in D21665 .
Without this fix, IP level fragmentation would occur.

Test Plan

Check with the following test program that no IP level fragmentation occurs (use RACK as the default stack):

connect-with-dst-opt.c917 BDownload