nvmet: fix mismatched serial

Target side uses 'bin2hex' to convert u64 serial number to a hex
string, saving serial number as be64 to keep right byte order.

Test on x86 server, config '0123456789abcdef' to 'attr_serial' on
target side, and run 'nvme id-ctrl /dev/nvme0' on initiator side,
then we can get the same SN string.

Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>

drivers/nvme/target/configfs.c

@@ -997,7 +997,7 @@ static ssize_t nvmet_subsys_attr_serial_show(struct config_item *item,
 {
 	struct nvmet_subsys *subsys = to_subsys(item);
 
-	return snprintf(page, PAGE_SIZE, "%llx\n", subsys->serial);
+	return snprintf(page, PAGE_SIZE, "%llx\n", be64_to_cpu(subsys->serial));
 }
 
 static ssize_t nvmet_subsys_attr_serial_store(struct config_item *item,
@@ -1009,7 +1009,7 @@ static ssize_t nvmet_subsys_attr_serial_store(struct config_item *item,
 		return -EINVAL;
 
 	down_write(&nvmet_config_sem);
-	to_subsys(item)->serial = serial;
+	to_subsys(item)->serial = cpu_to_be64(serial);
 	up_write(&nvmet_config_sem);
 
 	return count;