selftests/bpf: Fix nanosleep for real this time

Amazingly, some libc implementations don't call __NR_nanosleep syscall from
their nanosleep() APIs. Hammer it down with explicit syscall() call and never
get back to it again. Also simplify code for timespec initialization.

I verified that nanosleep is called w/ printk and in exactly same Linux image
that is used in Travis CI. So it should both sleep and call correct syscall.

v1->v2:
  - math is too hard, fix usec -> nsec convertion (Martin);
  - test_vmlinux has explicit nanosleep() call, convert that one as well.

Fixes: 4e1fd25d19 ("selftests/bpf: Fix usleep() implementation")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200314002743.3782677-1-andriin@fb.com

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

@@ -11,7 +11,7 @@ static void nsleep()
 {
 	struct timespec ts = { .tv_nsec = MY_TV_NSEC };
 
-	(void)nanosleep(&ts, NULL);
+	(void)syscall(__NR_nanosleep, &ts, NULL);
 }
 
 void test_vmlinux(void)