Linux /proc/spl/kstat per-dataset statistics do not account for file writes performed with mmap

[drowfx]

System information

Type	Version/Name
Distribution Name	Debian
Distribution Version	Bullseye (11)
Kernel Version	5.10.0-11-amd64
Architecture	x86_64
OpenZFS Version	zfs-2.1.2-1~bpo11+1

Describe the problem you're observing

When a file on a ZFS dataset is opened for write and mmaped (with PROT_WRITE), and the file is written via the memory mapping, the writes are not accounted in the /proc/spl/kstat/zfs/<pool>/objset-<id> file: Neither the nwritten nor the writes field increase.
The data is being written to the ZFS fs, being visible in the usedbydataset property.

Describe how to reproduce the problem

The python script here reproduces the issue.
It first writes 512 MiB to a test file using write(), checks the writes in kstat. Then it writes 512 MiB to another test file using mmap() and checks the writes in kstat again. The usedbydataset is also checked to verify the data is being written.
Running the script with a test dataset gives the following output:

Writing 512 MiB using write(2)...
After writing with write: kstat: writes 512 nwritten 536870912
dataset size 518987776

Writing 512 MiB using mmap(2)...
After writing with mmap: kstat: writes: 0 nwritten 0
dataset size 1037869056

As you can see, the write using mmap is not accounted in kstat, even though the dataset size increases by roughly the expected amount.

The expected result would be that at least 1 writes and roughly 536870912 bytes are accounted for both write paths.

Include any warning/errors/backtraces from the system logs

N/A