selftests/bpf: update send_signal to lower perf evemts frequency

Similar to commit [1] sample perf events less often in
test_send_signal_nmi(). This should reduce perf events throttling.

[1] 7015843 ("selftests/bpf: Fix send_signal test with nested CONFIG_PARAVIRT")

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20241112110906.3045278-5-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: eddyz87

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

@@ -229,7 +229,8 @@ static void test_send_signal_perf(bool signal_thread, bool remote)
 static void test_send_signal_nmi(bool signal_thread, bool remote)
 {
 	struct perf_event_attr attr = {
-		.sample_period = 1,
+		.freq = 1,
+		.sample_freq = 1000,
 		.type = PERF_TYPE_HARDWARE,
 		.config = PERF_COUNT_HW_CPU_CYCLES,
 	};