fuse: block request allocation until io-uring init is complete

Avoid races and block request allocation until io-uring
queues are ready.

This is a especially important for background requests,
as bg request completion might cause lock order inversion
of the typical queue->lock and then fc->bg_lock

    fuse_request_end
       spin_lock(&fc->bg_lock);
       flush_bg_queue
         fuse_send_one
           fuse_uring_queue_fuse_req
           spin_lock(&queue->lock);

Signed-off-by: Bernd Schubert <bernd@bsbernd.com>
Reviewed-by: Luis Henriques <luis@igalia.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>