libbpf-tools: fix biosnoop exit code

perf_buffer__poll returns a count or a negative error value. If the
loop is broken because of timeout err needs to be zero or else an
error is flagged. As the count may be non-zero, and this isn't an
error, move the setting of err to zero earlier. This also allows
switching a goto to a break.

libbpf-tools/biosnoop.c

@@ -334,10 +334,10 @@ int main(int argc, char **argv)
  fprintf(stderr, "error polling perf buffer: %s\n", strerror(-err));
  goto cleanup;
  }
- if (env.duration && get_ktime_ns() > time_end)
- goto cleanup;
  /* reset err to return 0 if exiting */
  err = 0;
+ if (env.duration && get_ktime_ns() > time_end)
+ break;
  }
 
 cleanup: