nvmet: don't report 0-bytes in serial number

nvmet: don't report 0-bytes in serial number

The NVME standard mandates that the SN, MN, and FR fields of the Identify
Controller Data Structure be "ASCII strings".  That means that they may
not contain 0-bytes, not even string terminators.

Signed-off-by: Martin Wilck <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
[hch: fixed for the move of the serial field, updated description]
Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>