Fixed #20946 -- model inheritance + m2m failure

Cleaned up the internal implementation of m2m fields by removing
related.py _get_fk_val(). The _get_fk_val() was doing the wrong thing
if asked for the foreign key value on foreign key to parent model's
primary key when child model had different primary key field.

tests/model_inheritance/models.py

@@ -162,3 +162,9 @@ class Mixin(object):
 
 class MixinModel(models.Model, Mixin):
     pass
+
+class Base(models.Model):
+    titles = models.ManyToManyField(Title)
+
+class SubBase(Base):
+    sub_id = models.IntegerField(primary_key=True)