The syncache entry holds one TCPS_SYN_RECEIVED count that normally is
transferred to the the newborn tp. Upon failure syncache_socket() shall
not use TCPSTATES_INC/TCPSTATES_DEC (see 5050df3f4aa4 why). But when
syncache_socket() fails in_pcbconnect(), it calls tcp_discardcb() to free
resources that were just allocated by tcp_newtcpcb() and this
tcp_discardcb() would do TCPSTATES_DEC(tp->t_state). The t_state is
TCPS_CLOSED at this point.
Make tcp_discardcb() symmetrical to tcp_newtcpcb() - not responsible for
the TCPSTATES. Make the caller responsible for state count book keeping.
Fixes: 3703e1a73e0e0367c04f47f793e46495e46e647b
MFC After: 2 weeks