tcp: plug an mbuf leak

When a challenge ACK should be sent via tcp_send_challenge_ack(),
but the rate limiter suppresses the sending, free the mbuf chain.
The caller of tcp_send_challenge_ack() expects this similar to the
callers of tcp_respond().

Approved by: so
Security: FreeBSD-SA-26:06.tcp
Security: CVE-2026-4247
Reviewed by: lstewart
Tested by: lstewart
Sponsored by: Netflix, Inc.