mlxsw: spectrum_router: Fix wrong kfree() in error path

The function allocates 'nhgi', not 'nh_grp', so it needs to free the
former in its error path.

Fixes: 7f7a417e ("mlxsw: spectrum_router: Split nexthop group configuration to a different struct")
Addresses-Coverity: ("Memory - corruptions  (USE_AFTER_FREE)")
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c

@@ -5423,7 +5423,7 @@ mlxsw_sp_nexthop6_group_info_init(struct mlxsw_sp *mlxsw_sp,
 		nh = &nhgi->nexthops[i];
 		mlxsw_sp_nexthop6_fini(mlxsw_sp, nh);
 	}
-	kfree(nh_grp);
+	kfree(nhgi);
 	return err;
 }