selftests: add a selftest for sctp vrf

This patch adds 12 small test cases: 01-04 test for the sysctl
net.sctp.l3mdev_accept. 05-10 test for only binding to a right
l3mdev device, the connection can be created. 11-12 test for
two socks binding to different l3mdev devices at the same time,
each of them can process the packets from the corresponding
peer. The tests run for both IPv4 and IPv6 SCTP.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>