Include l2asize in arcstat

For consistency with upstream pull in the l2asize update after
reworking it from Perl to Python.

References:

https://github.com/mharsch/arcstat/pull/11
https://github.com/mharsch/arcstat/pull/12

Signed-off-by: cburroughs <chris.burroughs@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2122