svcrpc: make xpo_recvfrom return only >=0

The only errors returned from xpo_recvfrom have been -EAGAIN and
-EAFNOSUPPORT.  The latter was removed by a previous patch.  That leaves
only -EAGAIN, which is treated just like 0 by the caller (svc_recv).

So, just ditch -EAGAIN and return 0 instead.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/svc_xprt.c

@@ -743,7 +743,7 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
 	svc_xprt_received(xprt);
 
 	/* No data, incomplete (TCP) read, or accept() */
-	if (len == 0 || len == -EAGAIN)
+	if (len <= 0)
 		goto out;
 
 	clear_bit(XPT_OLD, &xprt->xpt_flags);