Rewrite of function fBytes() in arc_summary.py

Replace if-elif-else construction with shorter loop;
remove unused parameter "Decimal"; centralize format
string; add function documentation string; conform to
PEP8.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Scot W. Stevenson <scot.stevenson@gmail.com>
Closes #6784