ebpf: add comment for some define in XDP filter

ebpf/xdp_filter.c

@@ -42,7 +42,10 @@
 /* Increase CPUMAP_MAX_CPUS if ever you have more than 64 CPUs */
 #define CPUMAP_MAX_CPUS     64
 
+/* Set it to 0 if for example you plan to use the XDP filter in a
+ * network card that don't support per CPU value (like netronome) */
 #define USE_PERCPU_HASH     1
+/* Set it to 0 if your XDP subsytem don't handle XDP_REDIRECT (like netronome) */
 #define GOT_TX_PEER         1
 
 /* set to non 0 to load balance in hardware mode on RSS_QUEUE_NUMBERS queues