tcp: refactor tcp_send_challenge_ack()

Refactor tcp_send_challenge_ack() such that the logic checking whether
a challenge ACK is sent or not is available in the separate function
tcp_challenge_ack_check(). This new function will also be used for
sending challenge ACKs in the SYN-cache code, which will be added in
upcoming commits.
No functional change intended.

Reviewed by: cc, Nick Banks, Peter Lei
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52717

(cherry picked from commit db37256ce5437e6c667a537afff0fd9f59576514)