Fix ZFS_READONLY implementation on Linux

[anodos325]

MS-FSCC 2.6 is the governing document for DOS attribute behavior.
It specifies the following:

For a file, applications can read the file butcannot write to it or delete it. For a directory,
applications cannot delete it, but applications can create and delete files from the directory.

Motivation and Context

Description

How Has This Been Tested?

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• 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.

[robn]

The fix looks right, but it took a moment to understand that you're just bringing it into alignment with the FreeBSD code. It'd be good for the commit message to reflect that.