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
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52840