Improve xfstests for direct-io

[padovan]

This issue is about:

• Adding tests for https://github.com/torvalds/linux/commit/41b21af388f94baf7433d4e7845703c7275251de to xfstests
• Enabling performance benchmarks with xfstests

Proposed by @krisman

[gctucker]

Adding the tests to xfstests would already be a very good target for this sprint. Then enabling xfstests to run in KernelCI can be done as a follow-up as described on #215 as it is non-trivial. Finally, dealing with performance regressions can be covered as part of #691 which is unlikely to get done as part of this sprint but could be considered for a future improvement.