net: document a side effect of ip_local_reserved_ports

If there is overlapp between ip_local_port_range and ip_local_reserved_ports with a huge reserved block, it will affect probability of selecting ephemeral ports, see file net/ipv4/inet_hashtables.c:723

    int __inet_hash_connect(
    ...
            for (i = 0; i < remaining; i += 2, port += 2) {
                    if (unlikely(port >= high))
                            port -= remaining;
                    if (inet_is_local_reserved_port(net, port))
                            continue;

    E.g. if there is reserved block of 10000 ports, two ports right after this block will be 5000 more likely selected than others.
    If this was intended, we can/should add note into documentation as proposed in this commit, otherwise we should think about different solution. One option could be mapping table of continuous port ranges. Second option could be letting user to modify step (port+=2) in above loop, e.g. using new sysctl parameter.

Signed-off-by: Otto Hollmann <otto.hollmann@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.rst

@@ -1073,7 +1073,9 @@ ip_local_reserved_ports - list of comma separated ranges
 
 	although this is redundant. However such a setting is useful
 	if later the port range is changed to a value that will
-	include the reserved ports.
+	include the reserved ports. Also keep in mind, that overlapping
+	of these ranges may affect probability of selecting ephemeral
+	ports which are right after block of reserved ports.
 
 	Default: Empty