tcp: apply rate limits to challenge ACKs

When sending challenge ACKs from the SYN-cache, apply the same rate
limiting as in other states.

Reviewed by: cc, rrs
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52754

(cherry picked from commit c2900b6e8255ba4f54dbd897cf42427db577ed3d)