selftests/bpf: Use sys_pidfd_open() helper when possible

SYS_pidfd_open may be undefined for old glibc, so using sys_pidfd_open()
helper defined in task_local_storage_helpers.h instead to fix potential
build failure.

And according to commit 7615d9e1780e ("arch: wire-up pidfd_open()"), the
syscall number of pidfd_open is always 434 except for alpha architure,
so update the definition of __NR_pidfd_open accordingly.

Signed-off-by: Hou Tao <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/prog_tests/bpf_iter.c

···
 #include <test_progs.h>
 #include <unistd.h>
 #include <sys/syscall.h>
+#include <task_local_storage_helpers.h>
 #include "bpf_iter_ipv6_route.skel.h"
 #include "bpf_iter_netlink.skel.h"
 #include "bpf_iter_bpf_map.skel.h"
···
 	bpf_iter_bpf_map__destroy(skel);
 }
 
-static int pidfd_open(pid_t pid, unsigned int flags)
-{
-	return syscall(SYS_pidfd_open, pid, flags);
-}
-
 static void check_bpf_link_info(const struct bpf_program *prog)
 {
 	LIBBPF_OPTS(bpf_iter_attach_opts, opts);
···
 	union bpf_iter_link_info linfo;
 	int pidfd;
 
-	pidfd = pidfd_open(getpid(), 0);
-	if (!ASSERT_GT(pidfd, 0, "pidfd_open"))
+	pidfd = sys_pidfd_open(getpid(), 0);
+	if (!ASSERT_GT(pidfd, 0, "sys_pidfd_open"))
 		return;
 
 	memset(&linfo, 0, sizeof(linfo));

tools/testing/selftests/bpf/task_local_storage_helpers.h

···
 #include <sys/types.h>
 
 #ifndef __NR_pidfd_open
+#ifdef __alpha__
+#define __NR_pidfd_open 544
+#else
 #define __NR_pidfd_open 434
+#endif
 #endif
 
 static inline int sys_pidfd_open(pid_t pid, unsigned int flags)