Make arc_summary.py and dbufstat.py compatible with python3

To make arc_summary.py and dbufstat.py compatible with python3
some minor fixes were required, this was done automatically by
2to3 -w arc_summary.py and 2to3 -w dbufstat.py.

Signed-off-by: Hajo Möller <dasjoe@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Reviewed-by: Richard Laager <rlaager@wiktel.com>