block/rnbd: introduce rnbd_access_modes

Add one new array (marked with __maybe_unused to prevent gcc warning about
"defined but not used" with W=1), then we can remove rnbd_access_mode_str
and rnbd-common.c accordingly.

Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20230524070026.2932-4-guoqing.jiang@linux.dev


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