Relax special_small_blocks restrictions

[amotin]

special_small_blocks is applied to blocks after compression, so it makes no sense to demand its values to be power of 2. At most they could be multiple of 512, but that would still buy us nothing, so lets allow them be any within SPA_MAXBLOCKSIZE.

Also special_small_blocks does not really need to depend on the set recordsize, enabled pool features or presence of special vdev. At worst in any of those cases it will just do nothing, so we should not complicate users lives by artificial limitations.

While there, polish comments for recordsize and volblocksize.

How Has This Been Tested?

Created a pool without special vdev and set special_small_blocks to odd value.

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.

[behlendorf]

Yeah, this makes sense. Restricting it to a power of two implies it's enforced before compression, which isn't the case. I can see how this would be confusing.