block:sed-opal: SED Opal keystore

Add read and write functions that allow SED Opal keys to stored
in a permanent keystore.

Signed-off-by: Greg Joyce <gjoyce@linux.vnet.ibm.com>
Reviewed-by: Jonathan Derrick <jonathan.derrick@linux.dev>
Link: https://lore.kernel.org/r/20231004201957.1451669-2-gjoyce@linux.vnet.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>