svc: Make svc_check_conn_limits xprt independent

The svc_check_conn_limits function only manipulates xprt fields. Change references
to svc_sock->sk_xprt to svc_xprt directly.

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>