[BUGFIX] displayWarningMessages uses wrong returnUrl

The warning in the "About Modules" module about an
insecure admin offers a link to edit this user. Following
this link and then closing the form again leads to a 404.
The reason is a wrong returnUrl in
BackendUtility:: displayWarningMessages.

Resolves: #44537
Releases: 6.1, 6.0
Change-Id: I41d361d8b7314ac0bace8b8a5979a6f69128657c
Reviewed-on: https://review.typo3.org/17543
Reviewed-by: Mattias Nilsson
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn