Add msize configuration for windows-side 9P client (=higher WSL2 disk speed)

[BarsMonster]

There is a continuous effort to improve performance of 9P connections in WSL2. If we can resolve it at least for bulk transfers - half of the problem would be gone. Key to this msize configuration on 9P client. On linux side msize is configurable in fstab, but it seems to be not configurable in windows 9P client.

Suboptimal msize might reduce transfer speed for large file transfer (currently I see 400Mb/s through 9P down from native 4.5Gb/s achievable under Linux in WSL2)

Solution
Registry config is added for 9P client on windows side. At least msize is configurable.

Describe alternatives you've considered
Increase of existing hardcoded msize to 1-32Mb.
Automatic msize? Or more documentation how 9P client works / configured.

Additional context
Impact of msize to performance: https://wiki.qemu.org/Documentation/9psetup#Performance_Considerations_.28msize.29
Here others set default values to 1-32Mb : https://github.com/lxc/lxd/issues/10219

[BarsMonster]

Tried to increase default msize on Linux Kernel side from 128kB to 2Mb - did not helped (\net\9p\client.c).
I afraid it's enforced on windows size.

[bbsan2k]

Is there any plan to implement this any time soon?