SUNRPC: select privileged port numbers at random

Make the RPC client select privileged ephemeral source ports at
random.  This improves DRC behavior on the server by using the
same port when reconnecting for the same mount point, but using
a different port for fresh mounts.

The Linux TCP implementation already does this for nonprivileged
ports.  Note that TCP sockets in TIME_WAIT will prevent quick reuse
of a random ephemeral port number by leaving the port INUSE until
the connection transitions out of TIME_WAIT.

Test plan:
Connectathon against every known server implementation using multiple
mount points.  Locking especially.
Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/xprtsock.c

@@ -930,6 +930,13 @@ static void xs_udp_timer(struct rpc_task *task)
 	xprt_adjust_cwnd(task, -ETIMEDOUT);
 }
 
+static unsigned short xs_get_random_port(void)
+{
+	unsigned short range = xprt_max_resvport - xprt_min_resvport;
+	unsigned short rand = (unsigned short) net_random() % range;
+	return rand + xprt_min_resvport;
+}
+
 /**
  * xs_set_port - reset the port number in the remote endpoint address
  * @xprt: generic transport
@@ -1275,7 +1282,7 @@ int xs_setup_udp(struct rpc_xprt *xprt, struct rpc_timeout *to)
 	memset(xprt->slot, 0, slot_table_size);
 
 	xprt->prot = IPPROTO_UDP;
-	xprt->port = xprt_max_resvport;
+	xprt->port = xs_get_random_port();
 	xprt->tsh_size = 0;
 	xprt->resvport = capable(CAP_NET_BIND_SERVICE) ? 1 : 0;
 	/* XXX: header size can vary due to auth type, IPv6, etc. */
@@ -1317,7 +1324,7 @@ int xs_setup_tcp(struct rpc_xprt *xprt, struct rpc_timeout *to)
 	memset(xprt->slot, 0, slot_table_size);
 
 	xprt->prot = IPPROTO_TCP;
-	xprt->port = xprt_max_resvport;
+	xprt->port = xs_get_random_port();
 	xprt->tsh_size = sizeof(rpc_fraghdr) / sizeof(u32);
 	xprt->resvport = capable(CAP_NET_BIND_SERVICE) ? 1 : 0;
 	xprt->max_payload = RPC_MAX_FRAGMENT_SIZE;