selftests/bpf: Fix flaky test_btf_id test

With previous patch, one of subtests in test_btf_id becomes
flaky and may fail. The following is a failing example:

  Error: #26 btf
  Error: #26/174 btf/BTF ID
    Error: #26/174 btf/BTF ID
    btf_raw_create:PASS:check 0 nsec
    btf_raw_create:PASS:check 0 nsec
    test_btf_id:PASS:check 0 nsec
    ...
    test_btf_id:PASS:check 0 nsec
    test_btf_id:FAIL:check BTF lingersdo_test_get_info:FAIL:check failed: -1

The test tries to prove a btf_id not available after the map is closed.
But btf_id is freed only after workqueue and a rcu grace period, compared
to previous case just after a rcu grade period.

To fix the flaky test, I added a kern_sync_rcu() after closing map and
before querying btf id availability, essentially ensuring a rcu grace
period in the kernel, which seems making the test happy.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>

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

@@ -4629,6 +4629,7 @@ static int test_btf_id(unsigned int test_num)
 
 	/* The map holds the last ref to BTF and its btf_id */
 	close(map_fd);
+	kern_sync_rcu();
 	map_fd = -1;
 	btf_fd[0] = bpf_btf_get_fd_by_id(map_info.btf_id);
 	if (CHECK(btf_fd[0] >= 0, "BTF lingers")) {