Add sunrpc compat define for xp_sock.

SunRPC is using xp_sock in SVCXPRT, while TI-RPC is using
xp_fd. Add a compatibility define.

Illumos has something similar for the non-kernel case.

Obtained from: linux-nfs project (git 0d94036c3a0d4c24d22bf6a8c40ac6625d972c29)

include/rpc/svc.h

@@ -89,6 +89,7 @@ enum xprt_stat {
  */
 typedef struct __rpc_svcxprt {
 	int		xp_fd;
+#define	xp_sock		xp_fd
 	u_short		xp_port;	 /* associated port number */
 	const struct xp_ops {
 	    /* receive incoming requests */