libspl: staticify buf and pagesize, rename aok to libspl_assert_ok

[nabijaczleweli]

Motivation and Context

Exporting names this short can easily cause nasty collisions with user code. aok, being the "abort or raze my disk" tunable, is particularly fun.

Description

See above.

How Has This Been Tested?

It builds, I guess?

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) – though apparently the only use of aok is for zdb, so it don't matter too much
• 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. – none apply, hopefully
• I have run the ZFS Test Suite with this change applied. – CI take my hand
• All commit messages are properly formatted and contain Signed-off-by.