libbpf: fix libbpf_print

With the recent print rework we now have the following problem:
pr_{warning,info,debug} expand to __pr which calls libbpf_print.
libbpf_print does va_start and calls __libbpf_pr with va_list argument.
In __base_pr we again do va_start. Because the next argument is a
va_list, we don't get correct pointer to the argument (and print noting
in my case, I don't know why it doesn't crash tbh).

Fix this by changing libbpf_print_fn_t signature to accept va_list and
remove unneeded calls to va_start in the existing users.

Alternatively, this can we solved by exporting __libbpf_pr and
changing __pr macro to (and killing libbpf_print):
{
	if (__libbpf_pr)
		__libbpf_pr(level, "libbpf: " fmt, ##__VA_ARGS__)
}

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/test_progs.c

@@ -1785,18 +1785,12 @@ static void test_task_fd_query_tp(void)
 }
 
 static int libbpf_debug_print(enum libbpf_print_level level,
-			      const char *format, ...)
+			      const char *format, va_list args)
 {
-	va_list args;
-	int ret;
-
 	if (level == LIBBPF_DEBUG)
 		return 0;
 
-	va_start(args, format);
-	ret = vfprintf(stderr, format, args);
-	va_end(args);
-	return ret;
+	return vfprintf(stderr, format, args);
 }
 
 static void test_reference_tracking()