nvmet: check sscanf value for subsys serial attr

For nvmet in configfs.c we check return values for all the sscanf()
calls. Add similar check into the nvmet_subsys_attr_serial_store().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>