Preserve parent_bay during device bulk import when tags are present

[bctiemann]

Fixes: #20114

When tags are present, the _save_m2m method of BaseModelForm causes the instance to be forcibly saved via f.save_form_data(self.instance, cleaned_data[f.name]). However, because we detect the presence of a device's parent device bay by stashing it on the instance prior to save:

netbox/netbox/dcim/forms/bulk_import.py

Lines 730 to 732 in ff97ab9

	# Set parent_bay reverse relationship
	if device_bay := self.cleaned_data.get('device_bay'):
	self.instance.parent_bay = device_bay

The _save_m2m behavior normally leaves parent_bay alone, but if tags are present, this intermediary save_form_data has the effect of regenerating the saved obj and clearing out the synthetic parent_bay attribute.

This fix ensures parent_bay survives this process by pulling it from the object_form.instance prior to calling the save chain, rather than assuming the result of save will still have parent_bay.