vxlan: add extack support for create and changelink

This patch adds extack coverage in vxlan link
create and changelink paths. Introduces a new helper
vxlan_nl2flags to consolidate flag attribute validation.

thanks to Johannes Berg for some tips to construct the
generic vxlan flag extack strings.
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/vxlan.h

@@ -453,4 +453,35 @@ vxlan_fdb_clear_offload(const struct net_device *dev, __be32 vni)
 }
 #endif
 
+static inline void vxlan_flag_attr_error(int attrtype,
+					 struct netlink_ext_ack *extack)
+{
+#define VXLAN_FLAG(flg) \
+	case IFLA_VXLAN_##flg: \
+		NL_SET_ERR_MSG_MOD(extack, \
+				   "cannot change " #flg " flag"); \
+		break
+	switch (attrtype) {
+	VXLAN_FLAG(TTL_INHERIT);
+	VXLAN_FLAG(LEARNING);
+	VXLAN_FLAG(PROXY);
+	VXLAN_FLAG(RSC);
+	VXLAN_FLAG(L2MISS);
+	VXLAN_FLAG(L3MISS);
+	VXLAN_FLAG(COLLECT_METADATA);
+	VXLAN_FLAG(UDP_ZERO_CSUM6_TX);
+	VXLAN_FLAG(UDP_ZERO_CSUM6_RX);
+	VXLAN_FLAG(REMCSUM_TX);
+	VXLAN_FLAG(REMCSUM_RX);
+	VXLAN_FLAG(GBP);
+	VXLAN_FLAG(GPE);
+	VXLAN_FLAG(REMCSUM_NOPARTIAL);
+	default:
+		NL_SET_ERR_MSG_MOD(extack, \
+				   "cannot change flag");
+		break;
+	}
+#undef VXLAN_FLAG
+}
+
 #endif