funccount: Do not prepopulate location cache

Avoiding the prepopulation of the location cache allows us
to get rid of the `zero()` call at the end of each interval,
which would hang the program at full CPU. Replaced the
prepopulation with a `lookup_or_init` and the `zero()` call
with a call to `clear()`.

tools/funccount.py

@@ -170,11 +170,9 @@ def load(self):
  trace_count_text = """
 int PROBE_FUNCTION(void *ctx) {
  FILTER
- u64 loc = LOCATION;
- u64 *val = counts.lookup(&loc); // prepopulated on Python side
- if (val) {
- (*val)++;
- }
+ u64 loc = LOCATION, zero = 0;
+ u64 *val = counts.lookup_or_init(&loc, &zero);
+ (*val)++;
  return 0;
 }
  """
@@ -200,11 +198,6 @@ def load(self):
  self.bpf = BPF(text=bpf_text,
  usdt_contexts=[self.usdt] if self.usdt else [])
 
- # Initialize all map entries to zero
- counts = self.bpf["counts"]
- for location, function in self.trace_functions.items():
- counts[counts.Key(location)] = counts.Leaf()
-
 class Tool(object):
  def __init__(self):
  examples = """examples:
@@ -267,7 +260,7 @@ def run(self):
  continue
  print("%-36s %8d" %
  (self.probe.trace_functions[k.value], v.value))
- counts.zero()
+ counts.clear()
 
  if exiting:
  print("Detaching...")