samples/bpf: Fix tracex2 error: No such file or directory

since commit c504e5c2("net: skb: introduce kfree_skb_reason()")
kfree_skb() is replaced by kfree_skb_reason() and kfree_skb() is set to
the inline function. So, we replace kprobe/kfree_skb with
kprobe/kfree_skb_reason to solve the tracex2 error.

 $ cd samples/bpf
 $ sudo ./tracex2
 libbpf: prog 'bpf_prog2': failed to create kprobe 'kfree_skb+0x0' perf event: No such file or directory
 ERROR: bpf_program__attach failed
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/tencent_0F0DAE84C0B3C42E0B550E5E9F47A9114D09@qq.com

samples/bpf/tracex2_kern.c

@@ -22,14 +22,14 @@ struct {
 /* kprobe is NOT a stable ABI. If kernel internals change this bpf+kprobe
  * example will no longer be meaningful
  */
-SEC("kprobe/kfree_skb")
+SEC("kprobe/kfree_skb_reason")
 int bpf_prog2(struct pt_regs *ctx)
 {
 	long loc = 0;
 	long init_val = 1;
 	long *value;
 
-	/* read ip of kfree_skb caller.
+	/* read ip of kfree_skb_reason caller.
 	 * non-portable version of __builtin_return_address(0)
 	 */
 	BPF_KPROBE_READ_RET_IP(loc, ctx);

samples/bpf/tracex2_user.c

@@ -146,7 +146,8 @@ int main(int ac, char **argv)
 	signal(SIGINT, int_exit);
 	signal(SIGTERM, int_exit);
 
-	/* start 'ping' in the background to have some kfree_skb events */
+	/* start 'ping' in the background to have some kfree_skb_reason
+	 * events */
 	f = popen("ping -4 -c5 localhost", "r");
 	(void) f;