nvme: strip trailing 0-bytes in wwid_show

nvme: strip trailing 0-bytes in wwid_show

Some broken controllers (such as earlier Linux targets) pad model or
serial fields with 0-bytes rather than spaces. The NVMe spec disallows
0 bytes in "ASCII" fields.  Thus strip trailing 0-bytes, too. Also make
sure that we get no underflow for pathological input.

Signed-off-by: Martin Wilck <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Reviewed-by: Keith Busch <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>