driver: tun: Use new macro SOCK_IOC_TYPE instead of literal number 0x89

The current codes use _IOC_TYPE(cmd) == 0x89 to check if the cmd is one
socket ioctl command like SIOCGIFHWADDR. But the literal number 0x89 may
confuse readers. So create one macro SOCK_IOC_TYPE to enhance the readability.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 9142db8..1588469 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1985,7 +1985,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 	int le;
 	int ret;
 
-	if (cmd == TUNSETIFF || cmd == TUNSETQUEUE || _IOC_TYPE(cmd) == 0x89) {
+	if (cmd == TUNSETIFF || cmd == TUNSETQUEUE || _IOC_TYPE(cmd) == SOCK_IOC_TYPE) {
 		if (copy_from_user(&ifr, argp, ifreq_len))
 			return -EFAULT;
 	} else {