nvmet: use strlcpy() instead of strcpy()

Although the code modified by this patch looks fine to me, this patch avoids
that Coverity reports the following complaint (ID 1364971 and ID 1364973):
"You might overrun the 256-character fixed-size string id->subnqn".

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>