Add dataset_kstats_update.. to mmap read/write paths

This allows reads/writes caused by accesses to mmap files to be
accounted correctly in the per-dataset kstats for both Linux and
FreeBSD.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Signed-off-by: Matthias Blankertz <matthias@blankertz.org>
Closes #12994
Closes #13044