block: sed-opal: ioctl for writing to shadow mbr

Allow modification of the shadow mbr. If the shadow mbr is not marked as
done, this data will be presented read only as the device content. Only
after marking the shadow mbr as done and unlocking a locking range the
actual content is accessible.

Co-authored-by: David Kozub <zub@linux.fjfi.cvut.cz>
Signed-off-by: Jonas Rabenstein <jonas.rabenstein@studium.uni-erlangen.de>
Signed-off-by: David Kozub <zub@linux.fjfi.cvut.cz>
Reviewed-by: Scott Bauer <sbauer@plzdonthack.me>
Reviewed-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>