shadow_copy2: Fix stream open for streams_depot paths

shadow_copy2: Fix stream open for streams_depot paths

streams_depot hands us absolute paths with : filename components
instead of having set smb_fname_in->stream_name.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15358

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Mon Apr 17 18:11:07 UTC 2023 on atb-devel-224

(cherry picked from commit 526f381f413d1cb5cde93b9542034f5ebfcfcc10)

Autobuild-User(v4-17-test): Jule Anger <janger@samba.org>
Autobuild-Date(v4-17-test): Tue Apr 18 16:22:04 UTC 2023 on sn-devel-184