Slide unlocking of the tcbinfo lock earlier in tcp_usr_send(), as it's
needed only for implicit connect cases. Under load, especially on SMP,
this can greatly reduce contention on the tcbinfo lock.

NB: Ambiguities about the state of so_pcb need to be resolved so that
all use of the tcbinfo lock in non-implicit connection cases can be
eliminated.

Submited by: Kazuaki Oda <kaakun at highway dot ne dot jp>