nvmet: fix space padding in serial number

[ Upstream commit c73996984902516745bc587d5e8a0b2e034aea05 ]

Commit 42de82a8b544 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: 42de82a8b544 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>