selftests: net/fcnal: Test --{force,no}-bind-key-ifindex

Test that applications binding listening sockets to VRFs without
specifying TCP_MD5SIG_FLAG_IFINDEX will work as expected. This would
be broken if __tcp_md5_do_lookup always made a strict comparison on
l3index. See this email:

https://lore.kernel.org/netdev/209548b5-27d2-2059-f2e9-2148f5a0291b@gmail.com/



Applications using tcp_l3mdev_accept=1 and a single global socket (not
bound to any interface) also should have a way to specify keys that are
only for the default VRF, this is done by --force-bind-key-ifindex
without otherwise binding to a device.

Signed-off-by: Leonard Crestez <cdleonard@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>