Send $user_id to is_super_admin() when checking to set primary_blog for a user...

Send $user_id to is_super_admin() when checking to set primary_blog for a user in wpmu_create_blog(). see #19127.

git-svn-id: https://core.svn.wordpress.org/trunk@19182 1a063a9b-81f0-0310-95a4-ce76da25c4cd