Update tuntap_linux.c

Let the kernel takes care of assigning the port ID of the netlink socket.

The netlink socket id uses the same namespace in all docker containers,
so if the userspace code assigns the nl_pid then there is a possiblity
of a collision, resulting in errors like:
    "Could not bind NETLINK socket: Address already in use [98]"

Cherry-picked from 0df939ed5b0a7013a73f2df78f6a805b5ec555b3 in ntop/n2n#1169

Thanks @sbrodeur

src/tuntap_linux.c

@@ -189,7 +189,6 @@ int tuntap_open (tuntap_dev *device,
     memset(&sa, 0, sizeof(sa));
     sa.nl_family = PF_NETLINK;
     sa.nl_groups = RTMGRP_LINK;
-    sa.nl_pid = getpid();
 
     memset(&msg, 0, sizeof(msg));
     msg.msg_name = &sa;