samples/bpf: Use getppid instead of getpgrp for array map stress

When cross-compiling the bpf sample map_perf_test for aarch64, I find that
__NR_getpgrp is undefined. This causes build errors. This syscall is deprecated
and requires defining __ARCH_WANT_SYSCALL_DEPRECATED. To avoid having to define
that, just use a different syscall (getppid) for the array map stress test.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Joel Fernandes <joelaf@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/map_perf_test_user.c

@@ -282,7 +282,7 @@ static void test_array_lookup(int cpu)
 
 	start_time = time_get_ns();
 	for (i = 0; i < max_cnt; i++)
-		syscall(__NR_getpgrp, 0);
+		syscall(__NR_getppid, 0);
 	printf("%d:array_lookup %lld lookups per sec\n",
 	       cpu, max_cnt * 1000000000ll * 64 / (time_get_ns() - start_time));
 }