Should 400 if trying to set a block size on a local disk

Local disks have a hard-coded block size, so passing `block_size` at create time does nothing. Instead of ignoring it, we should error out at create time if a block size other than 4096 is passed in. https://github.com/oxidecomputer/omicron/blob/985304a607b16191bdc35e0ea87fa522d925b514/nexus/src/app/sagas/disk_create.rs#L327-L329 https://github.com/oxidecomputer/omicron/blob/985304a607b16191bdc35e0ea87fa522d925b514/nexus/src/app/disk.rs#L200-L202