opensnoop: fix --cgroupmap with kfunc

Commit c347fe6c9f75 ("Support kfunc in opensnoop.py") introduces an
alternative probe on do_sys_open() with kfuncs instead of kprobes. This
new implementation is used if the kernel supports it. But it removed the
--cgroupmap filter added in commit b2aa29fa3269 ("tools: cgroup
filtering in execsnoop/opensnoop").

This patch adds the --cgroupmap filter in the kfunc implementation.
diff --git a/tools/opensnoop.py b/tools/opensnoop.py
index 4f94d01..b28d7d5 100755
--- a/tools/opensnoop.py
+++ b/tools/opensnoop.py
@@ -177,6 +177,12 @@
     PID_TID_FILTER
     UID_FILTER
     FLAGS_FILTER
+#if CGROUPSET
+    u64 cgroupid = bpf_get_current_cgroup_id();
+    if (cgroupset.lookup(&cgroupid) == NULL) {
+      return 0;
+    }
+#endif
 
     struct data_t data = {};
     bpf_get_current_comm(&data.comm, sizeof(data.comm));