selftests/net: convert sctp_vrf.sh to run it in unique namespace

Here is the test result after conversion.

]# ./sctp_vrf.sh
Testing For SCTP VRF:
TEST 01: nobind, connect from client 1, l3mdev_accept=1, Y [PASS]
...
TEST 12: bind vrf-2 & 1 in server, connect from client 1 & 2, N [PASS]
***v6 Tests Done***

Acked-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>