add ARC & L2ARC hit/miss bytes to arcstat

[tkittich]

Motivation and Context

Currently there's no way to see the actual bytes hit/miss from the ARC & L2ARC.

Description

This PR adds number of bytes that were hit/miss from ARC & L2ARC to arcstat.

How Has This Been Tested?

Tested and working in a local VM. Not sure if there're any corner cases though.

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.

[amotin]

It would be nice if hits and hit_bytes (and similarly misses) were accounted together and same way. And you've completely missed iohits. And for L2ARC it seems hit_bytes duplicates read_bytes.