selftests/bpf: fix a clang compilation error for send_signal.c

Building selftests/bpf with latest clang compiler (clang15 built
from source), I hit the following compilation error:
  /.../prog_tests/send_signal.c:43:16: error: variable 'j' set but not used [-Werror,-Wunused-but-set-variable]
                  volatile int j = 0;
                               ^
  1 error generated.
The problem also exists with clang13 and clang14. clang12 is okay.

In send_signal.c, we have the following code
  volatile int j = 0;
  ...
  for (int i = 0; i < 100000000 && !sigusr1_received; i++)
    j /= i + 1;
to burn cpu cycles so bpf_send_signal() helper can be tested
in nmi mode.

Slightly changing 'j /= i + 1' to 'j /= i + j' or 'j++' can
fix the problem. Further investigation indicated this should be
a clang bug ([1]). The upstream fix will be proposed later. But it is
a good idea to workaround the issue to unblock people who build
kernel/selftests with clang.

 [1] https://discourse.llvm.org/t/strange-clang-unused-but-set-variable-error-with-volatile-variables/60841

Signed-off-by: Yonghong Song <yhs@fb.com>

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

@@ -65,7 +65,7 @@ static void test_send_signal_common(struct perf_event_attr *attr,
 
 		/* wait a little for signal handler */
 		for (int i = 0; i < 100000000 && !sigusr1_received; i++)
-			j /= i + 1;
+			j /= i + j;
 
 		buf[0] = sigusr1_received ? '2' : '0';
 		ASSERT_EQ(sigusr1_received, 1, "sigusr1_received");