samples/bpf: xdpsock, minor fixes

- xsks_map size was fixed to 4, changed it MAX_SOCKS
- Remove redundant definition of MAX_SOCKS in xdpsock_user.c
- In dump_stats(), add NULL check for xsks[i]
Signed-off-by: NPrashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: NBjörn Töpel <bjorn.topel@intel.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

samples/bpf/xdpsock_kern.c

@@ -16,7 +16,7 @@ struct bpf_map_def SEC("maps") xsks_map = {
 	.type = BPF_MAP_TYPE_XSKMAP,
 	.key_size = sizeof(int),
 	.value_size = sizeof(int),
-	.max_entries = 4,
+	.max_entries = MAX_SOCKS,
 };
 
 struct bpf_map_def SEC("maps") rr_map = {

samples/bpf/xdpsock_user.c

@@ -118,7 +118,6 @@ struct xdpsock {
 	unsigned long prev_tx_npkts;
 };
 
-#define MAX_SOCKS 4
 static int num_socks;
 struct xdpsock *xsks[MAX_SOCKS];
 
@@ -596,7 +595,7 @@ static void dump_stats(void)
 
 	prev_time = now;
 
-	for (i = 0; i < num_socks; i++) {
+	for (i = 0; i < num_socks && xsks[i]; i++) {
 		char *fmt = "%-15s %'-11.0f %'-11lu\n";
 		double rx_pps, tx_pps;