Prefix table name against the post_parent field in...

Prefix table name against the post_parent field in _edit_attachments_query_helper() to avoid ambiguous queries. props benbalter. fixes #19779.

git-svn-id: https://core.svn.wordpress.org/trunk@19720 1a063a9b-81f0-0310-95a4-ce76da25c4cd