svc: Change the svc_sock in the rqstp structure to a transport

The rqstp structure contains a pointer to the transport for the
RPC request. This functionaly trivial patch adds an unamed union
with pointers to both svc_sock and svc_xprt. Ultimately the
union will be removed and only the rq_xprt field will remain. This
allows incrementally extracting transport independent interfaces without
one gigundo patch.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Acked-by: Neil Brown <neilb@suse.de>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Greg Banks <gnb@sgi.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>