Make zdb -R scale less poorly

[rincebrain]

zdb -R with :d tries to use gzip decompression 9 times per size. There's absolutely no reason for that, they're all the same decompressor.

Motivation and Context

When I was looking at a block that wouldn't decompress, I noticed I was trying gzip 9 times per size. This seemed quite silly.

Also, as I was doing it, I swear I wrote this before, but I don't see any PR with me having done it in quickly looking, so here we are.

Description

Mask off the other gzip levels, you can't tell the difference with decompression anyway, so trying 9 times never produced more information.

How Has This Been Tested?

It ran.

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.