[stable19] Change free space calculation

[backportbot-nextcloud]

⚠️ This backport had conflicts and is incomplete ⚠️

backport of #22528

[faily-bot]

🤖 beep boop beep 🤖

Here are the logs for the failed build:

Status of 32578: failure

jsunit

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PhantomJS 2.1.1 (Linux 0.0.0) LOG: 'JQMIGRATE: Migrate is installed, version 1.4.1'
PhantomJS 2.1.1 (Linux 0.0.0) DEBUG: 'OCA.Files.Settings initialized'
PhantomJS 2.1.1 (Linux 0.0.0) DEBUG: 'OCA.Files.Sidebar initialized'
PhantomJS 2.1.1 (Linux 0.0.0) DEBUG: 'OCA.Sharing.ShareSearch initialized'
PhantomJS 2.1.1 (Linux 0.0.0) DEBUG: 'OCA.Sharing.ExternalLinkActions initialized'
PhantomJS 2.1.1 (Linux 0.0.0): Executed 851 of 855 (skipped 4) SUCCESS (13.387 secs / 12.866 secs)

mariadb10.1-php7.2

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There were 2 warnings:
1) Test\Files\ViewTest::testRenameFailDeleteTargetKeepSource
Trying to configure method "writeStream" which cannot be configured because it does not exist, has not been specified, is final, or is static
2) Test\Files\ViewTest::testCopyFailDeleteTargetKeepSource
Trying to configure method "writeStream" which cannot be configured because it does not exist, has not been specified, is final, or is static
--
There was 1 failure:
1) Test\Files\Storage\Wrapper\QuotaTest::testFreeSpaceWithUnknownDiskSpace
Failed asserting that -1 matches expected 6.
/drone/src/tests/lib/Files/Storage/Wrapper/QuotaTest.php:92