Syncoockies can be used in combination with the syncache. If the cache
overflows, syncookies are used.
This patch restricts the usage of syncookies in this case: accept
syncookies only if there was an overflow of the syncache recently.
This mitigates a problem reported in PR217637, where is syncookie was
accepted without any recent drops.
Thanks to glebius@ for suggesting an improvement.

PR: 217637
Reviewed by: gnn, glebius
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D10272