arcstat: prevent ZeroDivisionError when L2ARC becomes empty

[ixhamza]

Motivation and Context

Prevent a ZeroDivisionError in arcstat when the L2ARC exists but becomes empty during monitoring.

Description

When monitoring L2ARC usage with arcstat, if the L2ARC device is present but its size l2_asize drops to zero, arcstat crashes with a ZeroDivisionError during percentage calculation:

$ arcstat -f time,l2size 1

Traceback (most recent call last):
  File "/bin/arcstat", line 802, in <module>
    main()
  File "/bin/arcstat", line 786, in main
    calculate()
  File "/bin/arcstat", line 754, in calculate
    v["l2pref%"] = 100 * v["l2pref"] / v["l2asize"]
                   ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~
ZeroDivisionError: division by zero

How Has This Been Tested?

• Manually verified that arcstat no longer crashes when L2ARC becomes empty during runtime.

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.

[ryao]

A nice follow-up to this would be to add a CodeQL check to detect potential ZeroDivisionError issues in Python code.