nvmet: fix space padding in serial number

Commit 42de82a8 previously attempted to fix this, and it did
correctly pad the MN and FR fields with spaces, but the SN field still
contains 0 bytes.  The current code fills out the first 16 bytes with
hex2bin, leaving the last 4 bytes zeroed.  Rather than adding a lot of
error-prone math to avoid overwriting SN twice, just set the whole thing
to spaces up front (it's only 20 bytes).

Fixes: 42de82a8 ("nvmet: don't report 0-bytes in serial number")
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>