tun: Consistently configure generic netdev params via rtnetlink

Configuring generic network device parameters on tun will fail in
presence of IFLA_INFO_KIND attribute in IFLA_LINKINFO nested attribute
since tun_validate() always return failure.

This can be visualized with following ip-link(8) command sequences:

# ip link set dev tun0 group 100
# ip link set dev tun0 group 100 type tun
RTNETLINK answers: Invalid argument

with contrast to dummy and veth drivers:

# ip link set dev dummy0 group 100
# ip link set dev dummy0 type dummy

# ip link set dev veth0 group 100
# ip link set dev veth0 group 100 type veth

Fix by returning zero in tun_validate() when @data is NULL that is
always in case since rtnl_link_ops->maxtype is zero in tun driver.

Fixes: f019a7a594d9 ("tun: Implement ip link del tunXXX")
Signed-off-by: Serhey Popovych <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tun.c

···
 static int tun_validate(struct nlattr *tb[], struct nlattr *data[],
 			struct netlink_ext_ack *extack)
 {
+	if (!data)
+		return 0;
 	return -EINVAL;
 }