samples/bpf: Fix incorrect use of strlen in xdp_redirect_cpu

Commit b599015f ("samples/bpf: Fix application of sizeof to pointer")
tried to fix a bug where sizeof was incorrectly applied to a pointer instead
of the array string was being copied to, to find the destination buffer size,
but ended up using strlen, which is still incorrect. However, on closer look
ifname_buf has no other use, hence directly use optarg.

Fixes: b599015f ("samples/bpf: Fix application of sizeof to pointer")
Fixes: e531a220 ("samples: bpf: Convert xdp_redirect_cpu to XDP samples helper")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Tested-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Link: https://lore.kernel.org/bpf/20211112020301.528357-1-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

samples/bpf/xdp_redirect_cpu_user.c

@@ -309,7 +309,6 @@ int main(int argc, char **argv)
 	const char *mprog_filename = NULL, *mprog_name = NULL;
 	struct xdp_redirect_cpu *skel;
 	struct bpf_map_info info = {};
-	char ifname_buf[IF_NAMESIZE];
 	struct bpf_cpumap_val value;
 	__u32 infosz = sizeof(info);
 	int ret = EXIT_FAIL_OPTION;
@@ -390,10 +389,10 @@ int main(int argc, char **argv)
 		case 'd':
 			if (strlen(optarg) >= IF_NAMESIZE) {
 				fprintf(stderr, "-d/--dev name too long\n");
+				usage(argv, long_options, __doc__, mask, true, skel->obj);
 				goto end_cpu;
 			}
-			safe_strncpy(ifname_buf, optarg, strlen(ifname_buf));
-			ifindex = if_nametoindex(ifname_buf);
+			ifindex = if_nametoindex(optarg);
 			if (!ifindex)
 				ifindex = strtoul(optarg, NULL, 0);
 			if (!ifindex) {