heads up on prefetch tunable in ZFS

[Kip Macy]

As far as I can tell systems that have less than 4GB are more often
hurt by prefetched than helped.  On i386 systems and systems with less
than 4GB, as of 193878 prefetch is now disabled by default. I've added
a prefetch enable tunable, to enable prefetching for those systems.
The prefetch disable tunable will continue to unconditionally disable
prefetching.

Cheers,
Kip