VFS: NULL dirfsp for openat on stream opens

VFS: NULL dirfsp for openat on stream opens

The main optimization is to avoid non_widelink_open() for streams
opens based on the fact that all streams opens are relative to
fsp->base_fsp, which is a pathref fsp already.

Neither streams_xattr nor streams_depot referenced dirfsp for the
streams case. Make this more obvious in the callers by passing NULL
and asserting this: non-streams opens and streams opens are just
different things, streams-opens can and do reference a base fsp and
don't need the non_widelink_open logic.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>