net: GRE: Add is_gretap_dev, is_ip6gretap_dev

Determining whether a device is a GRE device is easily done by
inspecting struct net_device.type. However, for the tap variants, the
type is just ARPHRD_ETHER.

Therefore introduce two predicate functions that use netdev_ops to tell
the tap devices.

Signed-off-by: Petr Machata <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Signed-off-by: Jiri Pirko <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/gre.h

···
 int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 		     bool *csum_err, __be16 proto, int nhs);
 
+bool is_gretap_dev(const struct net_device *dev);
+bool is_ip6gretap_dev(const struct net_device *dev);
+
 static inline int gre_calc_hlen(__be16 o_flags)
 {
 	int addend = 4;

net/ipv4/ip_gre.c

···
 	ip_tunnel_setup(dev, gre_tap_net_id);
 }
 
+bool is_gretap_dev(const struct net_device *dev)
+{
+	return dev->netdev_ops == &gre_tap_netdev_ops;
+}
+EXPORT_SYMBOL_GPL(is_gretap_dev);
+
 static int ipgre_newlink(struct net *src_net, struct net_device *dev,
 			 struct nlattr *tb[], struct nlattr *data[],
 			 struct netlink_ext_ack *extack)

net/ipv6/ip6_gre.c

···
 	netif_keep_dst(dev);
 }
 
+bool is_ip6gretap_dev(const struct net_device *dev)
+{
+	return dev->netdev_ops == &ip6gre_tap_netdev_ops;
+}
+EXPORT_SYMBOL_GPL(is_ip6gretap_dev);
+
 static bool ip6gre_netlink_encap_parms(struct nlattr *data[],
 				       struct ip_tunnel_encap *ipencap)
 {