selftests: forwarding: no_forwarding: Down ports on cleanup

[ Upstream commit e8497d6951ee8541d73784f9aac9942a7f239980 ]

This test neglects to put ports down on cleanup. Fix it.

Fixes: 476a4f05

 ("selftests: forwarding: add a no_forwarding.sh test")
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/0baf91dc24b95ae0cadfdf5db05b74888e6a228a.1724430120.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>