mlxsw: spectrum_router: Update prefix count for IPv6

The number of possible prefix lengths for IPv6 is 129 and not 128.

Fixes following warning from UBSAN when /128 routes are offloaded:

 UBSAN: Undefined behaviour in
drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c:2510:27 index 128 is out
of range for type 'long unsigned int [128]'

Fixes: 5e9c16cc ("mlxsw: spectrum_router: Implement private fib")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -307,7 +307,7 @@ static struct mlxsw_sp_rif *
 mlxsw_sp_rif_find_by_dev(const struct mlxsw_sp *mlxsw_sp,
 			 const struct net_device *dev);
 
-#define MLXSW_SP_PREFIX_COUNT (sizeof(struct in6_addr) * BITS_PER_BYTE)
+#define MLXSW_SP_PREFIX_COUNT (sizeof(struct in6_addr) * BITS_PER_BYTE + 1)
 
 struct mlxsw_sp_prefix_usage {
 	DECLARE_BITMAP(b, MLXSW_SP_PREFIX_COUNT);