tcp: refactor debug function syncookie_cmp()

• Don't bzero() the test structure. All fields checked are set by syncache_expand().
• Don't allocate TCP address logging string if there is nothing to report.
• Mark hash bucket argument as pointer to const.
• Make it void.

Differential Revision: https://reviews.freebsd.org/D50896

(cherry picked from commit e9e6a025b4523c9aa2885e892495601964e03056)