Fix zoneid when USER_NS is disabled

[Wraithh]

getzoneid() should return GLOBAL_ZONEID instead of 0 when USER_NS is disabled.

Motivation and Context

Fixes a Permission denied issue when creating or cloning a dataset when USER_NS is disabled as described in #15241.

Description

getzoneid() on Linux now returns GLOBAL_ZONEID instead of 0 when USER_NS is disabled (or getzoneid fails otherwise).

How Has This Been Tested?

This fixes the problem described in issue #15241

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.

[ryao]

Your commit message is missing a signed-off-by. Use git commit --amend -S to fix it and then git push -f origin issue-15241 to push the fix.

Other than that, this looks fine to me.