ipv6: Move gateway checks to a fib6_nh setting

The gateway setting is not per fib6_info entry but per-fib6_nh. Add a new
fib_nh_has_gw flag to fib6_nh and convert references to RTF_GATEWAY to
the new flag. For IPv6 address the flag is cheaper than checking that
nh_gw is non-0 like IPv4 does.

While this increases fib6_nh by 8-bytes, the effective allocation size of
a fib6_info is unchanged. The 8 bytes is recovered later with a
fib_nh_common change.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip6_fib.h

@@ -126,6 +126,7 @@ struct rt6_exception {
 
 struct fib6_nh {
 	struct in6_addr		nh_gw;
+	bool			fib_nh_has_gw;
 	struct net_device	*nh_dev;
 	struct lwtunnel_state	*nh_lwtstate;