selftests/bpf: Fix usdt_400 test case

usdt_400 test case relies on compiler using the same arg spec for
usdt_400 USDT. This assumption breaks with Clang (Clang generates
different arg specs with varying offsets relative to %rbp), so simplify
this further and hard-code the constant which will guarantee that arg
spec is the same across all 400 inlinings.

Fixes: 630301b0 ("selftests/bpf: Add basic USDT selftests")
Reported-by: Mykola Lysenko <mykolal@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220513173703.89271-1-andrii@kernel.org

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

@@ -190,9 +190,7 @@ __weak void trigger_300_usdts(void)
 
 static void __always_inline f400(int x __attribute__((unused)))
 {
-	static int y;
-
-	STAP_PROBE1(test, usdt_400, y++);
+	STAP_PROBE1(test, usdt_400, 400);
 }
 
 /* this time we have 400 different USDT call sites, but they have uniform
@@ -299,7 +297,7 @@ static void subtest_multispec_usdt(void)
 	trigger_400_usdts();
 
 	ASSERT_EQ(bss->usdt_100_called, 400, "usdt_400_called");
-	ASSERT_EQ(bss->usdt_100_sum, 399 * 400 / 2, "usdt_400_sum");
+	ASSERT_EQ(bss->usdt_100_sum, 400 * 400, "usdt_400_sum");
 
 cleanup:
 	test_usdt__destroy(skel);