smbd: Use share_mode_data->num_share_modes as a boolean

smbd: Use share_mode_data->num_share_modes as a boolean

This is a micro-commit showing that we don't actually need
share_mode_data->num_share_modes as a number *counting* the share mode
entries in share_entries.tdb anymore. Instead, we are only using it as
an indication for share_mode_lock_destructor() to see whether share
entries are around or not, i.e. whether it's worth keeping or deleting
the record in locking.tdb.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>