rnbd-srv: make process_msg_close returns void

Change the return type to void given it always returns 0.

Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Acked-by: Md Haris Iqbal <haris.iqbal@ionos.com>
Link: https://lore.kernel.org/r/20220902100055.25724-3-guoqing.jiang@linux.dev


Signed-off-by: Jens Axboe <axboe@kernel.dk>