SUNRPC: Clear rq_xid when receiving a new RPC Call

This is an eye-catcher for tracepoints that record the XID: it means
svc_rqst() has not received a full RPC Call with an XID yet.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svc_xprt.c

@@ -701,6 +701,8 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
 	arg->page_len = (pages-2)*PAGE_SIZE;
 	arg->len = (pages-1)*PAGE_SIZE;
 	arg->tail[0].iov_len = 0;
+
+	rqstp->rq_xid = xdr_zero;
 	return 0;
 }