Make syncookie_mac() use 'tcp_seq irs' in computing hash.
This fixes what seems like a simple oversight when the function was added in
r253210.

Reported by: Daniel Borkmann <dborkman@redhat.com>

Florian Westphal <fw@strlen.de>

Differential Revision: https://reviews.freebsd.org/D1628
Reviewed by: gnn
MFC after: 1 month
Sponsored by: Limelight Networks