Avoid error in cleanup if test if unsupported

[sethtroisi]

Motivation and Context

I built and tested zfs on a clean machine (not VM) with ubuntu 24.04 and got a failure in user_namespace/user_namespace_004, looking at the logs this test should be skipped on my machine but because the test setup (log_onexit with log_must zfs destroy) the test FAILs.

Description

Moved log_onexit below the check to see this test is supported.

log_onexit almost always appears as the 1st or 2nd real line in the file. I could instead add a variable SHOULD_CLEANUP=0, set it true after the log_unsupported check, and conditionally perform the cleanup in user_ns_cleanup but that felt less clean IMHO.

How Has This Been Tested?

Tested with ./scripts/zfs-tests.sh -v -t tests/zfs-tests/tests/functional/user_namespace/user_namespace_004 after my change SKIP 1 before my change FAIL 1

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.