smbd: let unix_convert() fail early if initial stat fails with EACCES

smbd: let unix_convert() fail early if initial stat fails with EACCES

Doing directory scans on the path components is not going to change this, so
give up early. No change in behaviour, as we would just fail later in
get_real_filename() otherwise.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Apr 22 21:08:39 UTC 2020 on sn-devel-184