Use get_current_user_id() instead of $user_ID in wp-admin/user-edit.php

[kovshenin]

I spent around an hour trying to figure out the difference between the user_id and the checkuser_id fields in user-edit.php, and then it hit me -- the latter uses the $user_ID global, with the uppercase ID, while everything else in the file uses $user_id which is the ID of the user being edited. What a nightmare!

[nacin]

In 27069:

Use get_current_user_id() instead of $user_ID in user-edit.php.

props kovshenin.
fixes #26274.