ZTS: default to random data in fill_fs

[behlendorf]

Motivation and Context

Switch to using a random fill by default for fill_fs users to restore traditional fill behavior. This may also help reduce some of the spurious failures we've seen with these test cases.

Description

Update the fill_fs helper function to request a random fill pattern when the "data" argument isn't specified. This ensures the default behavior is to perform a more realistic fill of incompressible blocks.

Additionally, update a few test cases to request a random fill.

How Has This Been Tested?

It has not. We'll see how it does in the CI.

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)