Adjust prefetch parameters.

• Reduce maximum prefetch distance for 32bit platforms to 8MB as it

was previously. Those systems didn't grow much probably, so better
stay conservative there.

• Retire array_rd_sz tunable, blocking prefetch for large requests.

We should not penalize applications trying to be more efficient. The
speculative prefetcher by itself has reasonable distance limits, and
1MB is not much at all these days.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15072