[tools/syscount.py]: fix total_ns wrong count (iovisor#1931)

It's useful to know each syscall's total latency
in that period, but not the latency of the last time
in that period.

Signed-off-by: Ahao Mu <[email protected]>

tools/syscount.py

@@ -481,7 +481,7 @@ def handle_errno(errstr):
 
  val = data.lookup_or_init(&key, &zero);
  val->count++;
- val->total_ns = bpf_ktime_get_ns() - *start_ns;
+ val->total_ns += bpf_ktime_get_ns() - *start_ns;
 #else
  u64 *val, zero = 0;
  val = data.lookup_or_init(&key, &zero);