Access the syncache secret directly from the V_tcp_syncache variable,
rather than indirectly through the backpointer to the tcp_syncache
structure stored in the hashtable bucket.

This also allows us to remove the requirement in syncookie_generate()
and syncookie_lookup() that the syncache hashtable bucket must be
locked.

Reviewed by: gallatin, rrs
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D21644