tcp: keep SYN-cache entry when sending of challenge ACK fails

Don't drop a SYN-cache entry just because a challenge ACK couldn't
be sent. This might only be a temporary failure.

Reviewed by: Nick Banks, glebius, jtl
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52840

(cherry picked from commit 7841b44f8491d69c75207d0f3a1eb34501d99edd)