ZTS: fix flakiness in cp_files_002_pos

[rrevans]

Motivation and Context

cp_files_002_pos fails 26 out of 500 times

The $RANDOM count argument to dd can be zero sometimes leading to spurious errors.

Description

Example failing log:

11:25:52.09 SUCCESS: zpool sync testpool
11:25:52.10 SUCCESS: dd if=/dev/urandom of=src.data bs=2048 seek=240 count=0 conv=notrunc
11:25:52.13 ERROR: cp --reflink=always src.data dst.data unexpectedly exited 0

Overwriting with dd ... count=0 does not exercise any code, so then cp --reflink=always spuriously succeeds because it does not test anything.

The fix is to adjust sizes so that $RANDOM yielding zero does not affect the test outcome.

How Has This Been Tested?

Ran cp_files_002_pos 500 times with zero failures.

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.