staging: usbip: Fix IPv6 support in usbipd

getaddrinfo() leaves the order of the returned addrinfo structs
unspecified. On systems with bindv6only disabled (this is the default),
PF_INET6 sockets bind to IPv4, too. Thus, IPv6 support in usbipd was
broken when getaddrinfo returned first IPv4 and then IPv6 addrinfos, as
the IPv6 bind failed with EADDRINUSE.

This patch uses seperate sockets for IPv4 and IPv6 and sets IPV6_V6ONLY
on all IPv6 sockets. Two command line arguments, -4 and -6 were added to
manually select the socket family.

Signed-off-by: Tobias Polzer <tobias.polzer@fau.de>
Signed-off-by: Dominik Paulus <dominik.paulus@fau.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>