mv: mv file on compressed Btrfs to tmpfs

[oech3]

#9771 but mv only currently.

Test is missing (as tt requires root or unshare+ User NS sandbox).

[oech3]

I'm considering to add test something like unshare -rm sh -c "mount -t tmpfs tmpfs /mnt && mv a /mnt", (but at different PR if allowed

[sylvestre]

sure, thanks

[oech3]

Is calling external script from *.rs allowed. The testwill be bit complex.