Temporarily disable Direct IO by default

[behlendorf]

Motivation and Context

Prior to creating the 2.3 release branch disable direct IO by default. Direct IO will be disabled in the -RC1 tag, we expect to finishing integrating fixes for the known issues and enable it by default in -RC2.

Description

While some remaining issues are resolved with the recently merged Direct IO functionality disable it by default.

Outstanding issues: #16594, #16596, read-only-verify PR pending.

How Has This Been Tested?

Will be tested by the CI. The direct IO test cases are already aware of this tuning and should be skipped.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.