samples, bpf: Refactor pointer error check with libbpf

Current method of checking pointer error is not user friendly.
Especially the __must_check define makes this less intuitive.

Since, libbpf has an API libbpf_get_error() which checks pointer error,
this commit refactors existing pointer error check logic with libbpf.

Signed-off-by: Daniel T. Lee <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

samples/bpf/sampleip_user.c

···
 #include "perf-sys.h"
 #include "trace_helpers.h"
 
-#define __must_check
-#include <linux/err.h>
-
 #define DEFAULT_FREQ	99
 #define DEFAULT_SECS	5
 #define MAX_IPS		8192
···
 			return 1;
 		}
 		links[i] = bpf_program__attach_perf_event(prog, pmu_fd);
-		if (IS_ERR(links[i])) {
+		if (libbpf_get_error(links[i])) {
 			fprintf(stderr, "ERROR: Attach perf event\n");
 			links[i] = NULL;
 			close(pmu_fd);
···
 
 	snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);
 	obj = bpf_object__open_file(filename, NULL);
-	if (IS_ERR(obj)) {
+	if (libbpf_get_error(obj)) {
 		fprintf(stderr, "ERROR: opening BPF object file failed\n");
 		obj = NULL;
 		goto cleanup;

samples/bpf/trace_event_user.c

···
 #include "perf-sys.h"
 #include "trace_helpers.h"
 
-#define __must_check
-#include <linux/err.h>
-
 #define SAMPLE_FREQ 50
 
 static int pid;
···
 			goto all_cpu_err;
 		}
 		links[i] = bpf_program__attach_perf_event(prog, pmu_fd);
-		if (IS_ERR(links[i])) {
+		if (libbpf_get_error(links[i])) {
 			printf("bpf_program__attach_perf_event failed\n");
 			links[i] = NULL;
 			close(pmu_fd);
···
 		goto err;
 	}
 	link = bpf_program__attach_perf_event(prog, pmu_fd);
-	if (IS_ERR(link)) {
+	if (libbpf_get_error(link)) {
 		printf("bpf_program__attach_perf_event failed\n");
 		link = NULL;
 		close(pmu_fd);
···
 	}
 
 	obj = bpf_object__open_file(filename, NULL);
-	if (IS_ERR(obj)) {
+	if (libbpf_get_error(obj)) {
 		printf("opening BPF object file failed\n");
 		obj = NULL;
 		goto cleanup;

samples/bpf/xdp_redirect_cpu_user.c

···
 #include <time.h>
 #include <linux/limits.h>
 
-#define __must_check
-#include <linux/err.h>
-
 #include <arpa/inet.h>
 #include <linux/if_link.h>
 
···
 	}
 
 	link = bpf_program__attach_tracepoint(prog, tp_category, tp_name);
-	if (IS_ERR(link))
+	if (libbpf_get_error(link))
 		exit(EXIT_FAIL_BPF);
 
 	return link;