tcp: improve error handling in syncache_socket()

If syncache_socket() fails after calling tcp_newtcpcb(), the resources
allocated in tcp_newtcpcb() needs to be freed. Just call
tcp_discardcb() to do this.
Thanks to jtl for making me aware of the issue and proposing a fix.

Reviewed by: glebius, jtl, rscheff
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45749

(cherry picked from commit 3703e1a73e0e0367c04f47f793e46495e46e647b)