selftests/bpf: Use nanosleep tracepoint in perf buffer test

The perf buffer tests triggers trace with nanosleep syscall,
but monitors all syscalls, which results in lot of data in the
buffer and makes it harder to debug. Let's lower the trace
traffic and monitor just nanosleep syscall.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>

Author: Jiri Olsa

tools/testing/selftests/bpf/progs/test_perf_buffer.c

@@ -12,7 +12,7 @@ struct {
 	__type(value, int);
 } perf_buf_map SEC(".maps");
 
-SEC("tp/raw_syscalls/sys_enter")
+SEC("tp/syscalls/sys_enter_nanosleep")
 int handle_sys_enter(void *ctx)
 {
 	int cpu = bpf_get_smp_processor_id();