Fix traceback in arc_summary.py

Arc Summary not working on FreeNAS 11.U5/6 with the following error:

Traceback (most recent call last):
File "/usr/local/bin/arc_summary.py", line 9, in <module>
main()
File "/usr/local/www/freenasUI/tools/arc_summary.py", line 1207, in main
_call_all(Kstat)
File "/usr/local/www/freenasUI/tools/arc_summary.py", line 1163, in _call_all
unsub(Kstat)
File "/usr/local/www/freenasUI/tools/arc_summary.py", line 936, in _l2arc_summary
if int(arc["l2_arc_evicts"]['lock_retries']) + int(arc["l2_arc_evicts"]["reading"]) > 0:
ValueError: invalid literal for int() with base 10: '1.52k'