Fix memory allocation for the checksum benchmark

Allocation via kmem_cache_alloc() is limited to less then 4m for
some architectures.

This commit limits the benchmarks with the linear abd cache to 1m
on all architectures and adds 4m + 16m benchmarks via non-linear
abd_alloc().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Co-authored-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #13669
Closes #13670