selftests: net: kill smcrouted in the cleanup logic in amt.sh

The amt.sh requires smcrouted for multicasting routing.
So, it starts smcrouted before forwarding tests.
It must be stopped after all tests, but it isn't.

To fix this issue, it kills smcrouted in the cleanup logic.

Fixes: c08e8bae ("selftests: add amt interface selftest script")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>