[BUGFIX] Fix mountpoint overlay detection

This is a follow up to 963eb99e7d7f9ab3d4a50153412f0ed3b927b5e5.
The comparison for the doktype needs to be non strict because
the database row contains a string and the constant is an integer.

Releases: master
Resolves: #59382
Change-Id: I7727e6bfb8c3475602f576f6259824cf1c7d2ca3
Reviewed-on: http://review.typo3.org/35079
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>