[1.1.X] Fixed #11226 -- Corrected an validation edge case with m2m relations...

[1.1.X] Fixed #11226 -- Corrected an validation edge case with m2m relations between two models with the same class name. Thanks to pkoch for the report, and to Ramiro Morales for the patch.

Backport of r12489 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12596 bcc190cf-cafb-0310-a4f2-bffc1f526a37