samples/bpf: Fix application of sizeof to pointer

The coccinelle check report:
"./samples/bpf/xdp_redirect_cpu_user.c:397:32-38:
ERROR: application of sizeof to pointer"
Using the "strlen" to fix it.
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: David Yang <davidcomponentone@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211012111649.983253-1-davidcomponentone@gmail.com

samples/bpf/xdp_redirect_cpu_user.c

@@ -325,7 +325,6 @@ int main(int argc, char **argv)
 	int add_cpu = -1;
 	int ifindex = -1;
 	int *cpu, i, opt;
-	char *ifname;
 	__u32 qsize;
 	int n_cpus;
 
@@ -393,9 +392,8 @@ int main(int argc, char **argv)
 				fprintf(stderr, "-d/--dev name too long\n");
 				goto end_cpu;
 			}
-			ifname = (char *)&ifname_buf;
-			safe_strncpy(ifname, optarg, sizeof(ifname));
-			ifindex = if_nametoindex(ifname);
+			safe_strncpy(ifname_buf, optarg, strlen(ifname_buf));
+			ifindex = if_nametoindex(ifname_buf);
 			if (!ifindex)
 				ifindex = strtoul(optarg, NULL, 0);
 			if (!ifindex) {