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.
In addition, it also corrects a KASSERT and adds protection code to assure
that the IP header chain and the TCP head fit in the first fragment as
required by RFC 7112.

Reviewed by: rrs@
MFC after: 3 days
Sponsored by: Nertflix, Inc.
Differential Revision: https://reviews.freebsd.org/D21666