s3:smbd: let open_directory() also use fd_open_atomic() as reopen_from_procfd() fallback

s3:smbd: let open_directory() also use fd_open_atomic() as reopen_from_procfd() fallback

Calling fd_open_atomic() without O_CREAT is the same as calling
fd_openat() directly, so we can also use it to open an existing
directory.

In the next step we'll move the reopen_from_procfd() fallback logic to
a single helper function.

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