Add read/write fixed prep support
Add read/write fixed prep support

Also unify the setup, all the read/write variants can use the same
helper.

Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing