Fix multi-table inheritance with LinkFields

The fields LinkFields adds were getting added twice when inheriting from
a model with a LinkField. LinkField tries to prevent this by check if
the fields are already there, but it only checked local_fields. It needs
to check all model fields, including inherited fields.

164ea5f00b3db switched from using get_all_field_names to local_fields,
because get_all_field_names loads the appcache which can cause a
circular import. This commit uses get_fields_with_model, which does not
load the appcache, so it should be ok.