perf bpf-loader: use PTR_ERR_OR_ZERO inetead of return code

Use PTR_ERR_OR_ZERO() in bpf__setup_stdout() return code instead of open
coded equivalent.

Signed-off-by: Ding Xiang <dingxiang@cmss.chinamobile.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1536284082-23466-2-git-send-email-dingxiang@cmss.chinamobile.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>