net: ipv6: Introduce ip6_multipath_hash_policy()

In order to abstract away access to the
ipv6.sysctl.multipath_hash_policy variable, which is not available on
systems compiled without IPv6 support, introduce a wrapper function
ip6_multipath_hash_policy() that falls back to 0 on non-IPv6 systems.

Use this wrapper from mlxsw/spectrum_router instead of a direct
reference.
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -7031,7 +7031,7 @@ static void mlxsw_sp_mp4_hash_init(char *recr2_pl)
 
 static void mlxsw_sp_mp6_hash_init(char *recr2_pl)
 {
-	bool only_l3 = !init_net.ipv6.sysctl.multipath_hash_policy;
+	bool only_l3 = !ip6_multipath_hash_policy(&init_net);
 
 	mlxsw_sp_mp_hash_header_set(recr2_pl,
 				    MLXSW_REG_RECR2_IPV6_EN_NOT_TCP_NOT_UDP);

include/net/ipv6.h

@@ -888,6 +888,17 @@ static inline int ip6_default_np_autolabel(struct net *net)
 }
 #endif
 
+#if IS_ENABLED(CONFIG_IPV6)
+static inline int ip6_multipath_hash_policy(const struct net *net)
+{
+	return net->ipv6.sysctl.multipath_hash_policy;
+}
+#else
+static inline int ip6_multipath_hash_policy(const struct net *net)
+{
+	return 0;
+}
+#endif
 
 /*
  *	Header manipulation