[activities] Fix logic error in user menu blocking activity updates

Summary:
There's an attempt to block updates to activities whilst the popup menu
is showing.

In one of the two code paths for positioning the menu the block is set
but instead of releasing the block at the end of the exec locks it
again.

CCBUG: 335725

Test Plan:
On X11 changed activities with the popup menu near the bottom.
It still flickered like crazy whilst the menu is open (see bug report)
But now at least fixes itself after

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16101