smbd: add and use vfs_fget_dos_attributes()

smbd: add and use vfs_fget_dos_attributes()

Commit d71ef1365cdde47aeb3465699181656b0655fa04 caused a regression where the
creation date on streams wasn't updated anymore on the stream fsp.

By adding a simple wrapper vfs_fget_dos_attributes() that takes care of

- passing only the base_fsp to the VFS, so the VFS can be completely agnostic of
  all the streams related complexity like fake fds,

- propagating any updated btime from the base_fsp->fsp_name to the
  stream_fsp->fsp_name

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15126
MR: https://gitlab.com/samba-team/samba/-/merge_requests/2643

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>