nvmet: use memcpy_and_pad for identify sn/fr

This changes the earlier patch "nvmet: don't report 0-bytes
in serial number" to use the memcpy_and_pad() helper introduced
in a previous patch.

Signed-off-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: Sagi Grimberg <sagi@grimbeg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>