Improve xfstests for direct-io

[padovan]

This issue is about:

• Adding tests for torvalds/linux@41b21af 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.