nfp: flower: Use struct_size() helper in kmalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/net/ethernet/netronome/nfp/flower/tunnel_conf.c:359:25: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/netronome/nfp/flower/tunnel_conf.c

@@ -356,7 +356,7 @@ __nfp_tun_add_route_to_cache(struct list_head *route_list,
 			return 0;
 		}
 
-	entry = kmalloc(sizeof(*entry) + add_len, GFP_ATOMIC);
+	entry = kmalloc(struct_size(entry, ip_add, add_len), GFP_ATOMIC);
 	if (!entry) {
 		spin_unlock_bh(list_lock);
 		return -ENOMEM;