arcstat: Fix integer division with python3

The arcstat script requests compatibility with python2 and python3, but
PEP 238 modified the / operator and results in erroneous output when
run under python3.

This commit replaces instances of / with //, yielding the expected
result in both versions of Python.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Valmiky Arquissandas <foss@kayvlim.com>
Closes #12603