selftests/bpf: Replaces the usage of CHECK calls for ASSERTs in vmlinux

vmlinux.c uses the `CHECK` calls even though the use of ASSERT_ series
of macros is preferred in the bpf selftests.

This patch replaces all `CHECK` calls for equivalent `ASSERT_`
macro calls.

Signed-off-by: Yuran Pereira <yuran.pereira@hotmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/GV1PR10MB6563ED1023A2A3AEF30BDA5DE8BBA@GV1PR10MB6563.EURPRD10.PROD.OUTLOOK.COM

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: 4e1fd25d19e8 ("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

selftests/bpf: Add vmlinux.h selftest exercising tracing of syscalls

Add vmlinux.h generation to selftest/bpf's Makefile. Use it from newly added
test_vmlinux to trace nanosleep syscall using 5 different types of programs:
- tracepoint;
- raw tracepoint;
- raw tracepoint w/ direct memory reads (tp_btf);
- kprobe;
- fentry.

These programs are realistic variants of real-life tracing programs,
excercising vmlinux.h's usage with tracing applications.

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/20200313172336.1879637-5-andriin@fb.com