vxlan: fix dereference of nexthop group in nexthop update path

fix dereference of nexthop group in fdb nexthop group
update validation path.

Fixes: 1274e1cc ("vxlan: ecmp support for mac fdb entries")
Reported-by: Ido Schimmel <idosch@idosch.org>
Suggested-by: Ido Schimmel <idosch@idosch.org>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -881,13 +881,13 @@ static int vxlan_fdb_nh_update(struct vxlan_dev *vxlan, struct vxlan_fdb *fdb,
 			goto err_inval;
 		}
 
-		if (!nh->is_group || !nh->nh_grp->mpath) {
+		nhg = rtnl_dereference(nh->nh_grp);
+		if (!nh->is_group || !nhg->mpath) {
 			NL_SET_ERR_MSG(extack, "Nexthop is not a multipath group");
 			goto err_inval;
 		}
 
 		/* check nexthop group family */
-		nhg = rtnl_dereference(nh->nh_grp);
 		switch (vxlan->default_dst.remote_ip.sa.sa_family) {
 		case AF_INET:
 			if (!nhg->has_v4) {