Fix the name of the zfs_prefetch_disable parameter (#16319)

The ZFS module parameter name is zfs_prefetch_disable, not
zfs_disable_prefetch.

Signed-off-by: Peter Doherty <peterd@acranox.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>