smb: Rework anonymous handling to avoid EINVAL

After the recent samba change, the "Invalid Argument" error can be still
returned when anonymous login is requested even after the commit 747c7f6e.
This is because `smbc_setOptionNoAutoAnonymousLogin` is called after returning
from the `auth_callback` function (i.e. there is one redundant iteration).
Let's rework the handling a bit and call that immediately, which bypasses
the issue.

Fixes: https://gitlab.gnome.org/GNOME/gvfs/-/issues/619


(cherry picked from commit 8c7e7904)