SUNRPC: service request network namespace helper introduced

This is a cleanup patch - makes code looks simplier.
It replaces widely used rqstp->rq_xprt->xpt_net by introduced SVC_NET(rqstp).

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>