rsxx: Allow block size to be determined by configuration.

Previously, the block size was determined by whether or not
our Hardware could handle 512 byte accesses. Now, all of our
Hardware can handle 512 and 4096 block sizes.

This fix allows it to be user configurable.

Signed-off-by: Philip J Kelleher <pjk1939@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>