nvmet: Change return code of discard command if not supported

Execute discard command on block device that doesn't support it
should return success.
Returning internal error while using multi-path fails the path.

Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>