FUSE:init is not async

[Peter-JanGootzen]

This is a big problem virtionfs needs to execute asynchronous handshake requests in the init.
Currently it just sends the INIT FUSE completion before those are done. This can result in race conditions on boot.

An attempt to fix this was made in this commit, however this made random operations randomly break. Seems there is some timeout on FUSE:init or that commit was triggering UB.