SUNRPC: Teach server to use xprt_sock_sendmsg for socket sends

xprt_sock_sendmsg uses the more efficient iov_iter-enabled kernel
socket API, and is a pre-requisite for server send-side support for
TLS.

Note that svc_process no longer needs to reserve a word for the
stream record marker, since the TCP transport now provides the
record marker automatically in a separate buffer.

The dprintk() in svc_send_common is also removed. It didn't seem
crucial for field troubleshooting. If more is needed there, a trace
point could be added in xprt_sock_sendmsg().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>