fuse: abort on fatal signal during sync init

In the Linux kernel, the following vulnerability has been resolved:

fuse: abort on fatal signal during sync init

When sync init is used and the server exits for some reason (error, crash)

while processing FUSE_INIT, the filesystem creation will hang. The reason

is that while all other threads will exit, the mounting thread (or process)

will keep the device fd open, which will prevent an abort from happening.

This is a regression from the async mount case, where the mount was done

first, and the FUSE_INIT processing afterwards, in which case there's no

such recursive syscall keeping the fd open.