Replacing MD5 by SipHash improves the performance of the TCP time stamp
initialisation, which is important when the host is dealing with a
SYN flood.
This affects the computation of the initial TCP sequence number for
the client side.
This has been discussed with secteam@.

Reviewed by: gallatin@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D21616