metadata-ns: check if the received KDBUS_ITEM_CREDS was translated

Check if the received KDBUS_ITEM_CREDS was translated into the domain
user namespace.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
metadata: let kdbus_meta_size() modify the attach mask

kdbus_meta_size() and kdbus_meta_write() are always called in pairs,
so we only need to compare the namespace once, and rely on the caller
side to to submit the same set of flags to both calls.

Pass the mask as pointer to kdbus_meta_size() so attach flags can
be dropped on namespace mismatches.

Signed-off-by: Daniel Mack <daniel@zonque.org>
metadata: translate audit.loginuid into domain->user_namespace

Signed-off-by: Daniel Mack <daniel@zonque.org>
test-metadata: remove the metadata check since now we translate into domain userns

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
metadata: drop most items if the target namespace has hide_pid enabled

I doubt anyone will actually ever use this, but supporting it doesn't
hurt.

Signed-off-by: Daniel Mack <daniel@zonque.org>