Use KSTAT_TYPE_RAW instead of KSTAT_TYPE_READ

This change removed the dependence on the new KSTAT_TYPE_READ type, and
instead uses the KSTAT_TYPE_RAW type. Adding new types is not portable
to the other distributions, so it is best to refrain from this practice
and stick to the known types.

To maintain a useable /proc interface, even for raw types, the SPL added
formatting callbacks that can be registered to convert the data into a
readable text format. This patch makes use of these callbacks for
displaying the SPA's read history kstat.

Signed-off-by: Prakash Surya <surya1@llnl.gov>