Use lookup instead of delete in test_stackid

BPF programs aren't allowed to attach to these functions since deadlocks
may occur, thus the test would fail since no stack is collected. Use
update_elem to test instead.

tests/python/test_stackid.py

@@ -24,11 +24,11 @@ class TestStackid(unittest.TestCase):
     def test_simple(self):
         b = bcc.BPF(text="""
 #include <uapi/linux/ptrace.h>
-#include <linux/bpf.h>
+struct bpf_map;
 BPF_STACK_TRACE(stack_traces, 10240);
 BPF_HASH(stack_entries, int, int);
 BPF_HASH(stub);
-int kprobe__htab_map_delete_elem(struct pt_regs *ctx, struct bpf_map *map, u64 *k) {
+int kprobe__htab_map_lookup_elem(struct pt_regs *ctx, struct bpf_map *map, u64 *k) {
     int id = stack_traces.get_stackid(ctx, BPF_F_REUSE_STACKID);
     if (id < 0)
         return 0;
@@ -40,14 +40,14 @@ int kprobe__htab_map_delete_elem(struct pt_regs *ctx, struct bpf_map *map, u64 *
         stub = b["stub"]
         stack_traces = b["stack_traces"]
         stack_entries = b["stack_entries"]
-        try: del stub[stub.Key(1)]
+        try: x = stub[stub.Key(1)]
         except: pass
         k = stack_entries.Key(1)
         self.assertIn(k, stack_entries)
         stackid = stack_entries[k]
         self.assertIsNotNone(stackid)
         stack = stack_traces[stackid].ip
-        self.assertEqual(b.ksym(stack[0]), "htab_map_delete_elem")
+        self.assertEqual(b.ksym(stack[0]), "htab_map_lookup_elem")
 
 
 if __name__ == "__main__":