Small fix to prefetch ranges aggregation

When after #16022 adding new range we aggregate more than two
existing ranges, that should be very rare, only if several streams
overlap, we may need to zero not the last range, but some earlier.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16072