Fix TypeError: unorderable types: str() > int() in arc_summary.py

Fix TypeError: unorderable types: str() > int() in arc_summary.py

Running arc_summary.py with a l2arc cache device around produces
the following error:

  Traceback (most recent call last):
    File "/usr/bin/arc_summary.py", line 1148, in <module>
      main()
    File "/usr/bin/arc_summary.py", line 1144, in main
      page(Kstat)
    File "/usr/bin/arc_summary.py", line 724, in _l2arc_summary
      arc["l2_arc_evicts"]["reading"] > 0:
  TypeError: unorderable types: str() > int()

This is due to arc["l2_arc_evicts"]['lock_retries'] and
arc["l2_arc_evicts"]["reading"] both being strings, returned
from fHits() earlier. Rather than adding them up and checking
if the result is > 0, this checks if either string is != '0'.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5538