nvmet: don't overwrite identify sn/fr with 0-bytes

The merged version of my patch "nvmet: don't report 0-bytes in serial
number" fails to remove two lines which should have been replaced,
so that the space-padded strings are overwritten again with 0-bytes.
Fix it.

Fixes: 42de82a8 nvmet: don't report 0-bytes in serial number
Signed-off-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: Sagi Grimberg <sagi@grimbeg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>