checkpatch job doesn't fail if Signed-off-by tags are missing

By design the checkpatch job is allowed to fail, but some checks must always generate a fail.

Specifically if the Signed-off-by line is missing, such as in this test:

https://gitlab.freedesktop.org/linux-media/users/hverkuil/-/jobs/60633606

It should also fail if:

1. A Change-Id tag is present,
2. there is a Cc to linux-media, or to any of the core committers.