mlxsw: spectrum_router: Replace 0-length array with flexible array

Zero-length arrays are deprecated[1]. Replace struct
mlxsw_sp_nexthop_group_info's "nexthops" 0-length array with a flexible
array. Detected with GCC 13, using -fstrict-flex-arrays=3:

drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c: In function 'mlxsw_sp_nexthop_group_hash_obj':
drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c:3278:38: warning: array subscript i is outside array bounds of 'struct mlxsw_sp_nexthop[0]' [-Warray-bounds=]
 3278 |                         val ^= jhash(&nh->ifindex, sizeof(nh->ifindex), seed);
      |                                      ^~~~~~~~~~~~
drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c:2954:33: note: while referencing 'nexthops'
 2954 |         struct mlxsw_sp_nexthop nexthops[0];
      |                                 ^~~~~~~~

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Cc: Ido Schimmel <idosch@nvidia.com>
Cc: Petr Machata <petrm@nvidia.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Tested-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -2951,7 +2951,7 @@ struct mlxsw_sp_nexthop_group_info {
 	   gateway:1, /* routes using the group use a gateway */
 	   is_resilient:1;
 	struct list_head list; /* member in nh_res_grp_list */
-	struct mlxsw_sp_nexthop nexthops[0];
+	struct mlxsw_sp_nexthop nexthops[];
 #define nh_rif	nexthops[0].rif
 };