backend: better NULL termination fix for IO scheduler switching

Signed-off-by: Jens Axboe <axboe@fb.com>
server: sanity check incoming command size (and payload)

Signed-off-by: Jens Axboe <axboe@fb.com>
server: ensure that incoming iolog name is NULL terminated

Signed-off-by: Jens Axboe <axboe@fb.com>