Ensure arc_size_break is filled in arc_summary.py

Ensure arc_size_break is filled in arc_summary.py

Use mfu_size and mru_size pulled from the arcstats
kstat file to calculate the mfu and mru percentages
for arc size breakdown.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: AndCycle <andcycle@andcycle.idv.tw>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5526
Closes #6770