tcp_usr_ready() shouldn't honor SBS_CANTSENDMORE, since data already
is in the socket and we must send it.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Fix three critical bugs when sbavail() was used in place of sbused(),
and one non-critical. The critical ones led to FIN bit sent before
remainer of data.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.