make_dev_s() sets si_drv1 before the node is published in devfs, which
avoids a race where cuse_client_open() could see it as NULL. It also now
reports finer-grained errors on failure, instead of only ENOMEM.
While here, drop the NULL checks on kern_dev in cuse_server_free_dev(),
since a device is only added to the server's list once it has been
created.
MFC after: 1 week
Sponsored by: The FreeBSD Foundation