Multisite in a subdirectory: For subdomain installs, use a root admin cookie...

Multisite in a subdirectory: For subdomain installs, use a root admin cookie path, not a wp-admin specific one.

This is necessary because, like subdirectory installs, we will have wp-admin accessed at different levels.

fixes #22705.



git-svn-id: https://core.svn.wordpress.org/trunk@23005 1a063a9b-81f0-0310-95a4-ce76da25c4cd