smbd: handle fake file handles in fdos_mode()

smbd: handle fake file handles in fdos_mode()

This ensures SMB requests on the quote fake file "$Extend/$Quota" don't hit the
VFS, where specifically in vfs_gpfs we log an error message if we fail to read
the DOS attributes for a file with

  vfs_gpfs_get_dos_attributes: Getting winattrs failed for $Extend/$Quota

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14731

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>