MINOR: raw-sock: always check for CO_FL_SOCK_WR_SH before sending

MINOR: raw-sock: always check for CO_FL_SOCK_WR_SH before sending

The test was added before splice() and send() to make sure we never
accidently send after a shutdown, because upper layers do not all
check and it's not their job to do it. In such a case we also set
errno to EPIPE so that the error can be accurately reported, e.g.,
in health checks.