uflow: convert bytes to str

Python 3 fix, similar to commit 99d1468 and commit 4e4c9e0 for ucalls.

Closes iovisor#1996.

tools/lib/uflow.py

@@ -196,7 +196,9 @@ def print_event(cpu, data, size):
  direction = "<- " if event.depth & (1 << 63) else "-> "
  print("%-3d %-6d %-6d %-8.3f %-40s" % (cpu, event.pid >> 32,
  event.pid & 0xFFFFFFFF, time.time() - start_ts,
- (" " * (depth - 1)) + direction + event.clazz + "." + event.method))
+ (" " * (depth - 1)) + direction + \
+ event.clazz.decode('utf-8', 'replace') + "." + \
+ event.method.decode('utf-8', 'replace')))
 
 bpf["calls"].open_perf_buffer(print_event)
 while 1: