perf_reader: allow more than one reader to poll

perf_reader: allow more than one reader to poll

When more than one events are used with attach_kprobe, the
perf_reader_poll() will segfault by buffer overflow like below.

*** buffer overflow detected ***: python terminated
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x7338f)[0x7fcc971fb38f]
/lib/x86_64-linux-gnu/libc.so.6(__fortify_fail+0x5c)[0x7fcc97292c9c]
/lib/x86_64-linux-gnu/libc.so.6(+0x109b60)[0x7fcc97291b60]
/lib/x86_64-linux-gnu/libc.so.6(+0x10ac04)[0x7fcc97292c04]
/usr/lib/x86_64-linux-gnu/libbcc.so(perf_reader_poll+0x42)[0x7fcc93969cc2]
/usr/lib/x86_64-linux-gnu/libffi.so.6(ffi_call_unix64+0x4c)[0x7fcc95c2badc]
/usr/lib/x86_64-linux-gnu/libffi.so.6(ffi_call+0x1fc)[0x7fcc95c2b40c]
/usr/lib/python2.7/lib-dynload/_ctypes.x86_64-linux-gnu.so(_ctypes_callproc+0x48e)[0x7fcc95e425fe]
/usr/lib/python2.7/lib-dynload/_ctypes.x86_64-linux-gnu.so(+0x15f9e)[0x7fcc95e43f9e]
python(PyEval_EvalFrameEx+0x965)[0x499be5]
python(PyEval_EvalCodeEx+0x2ac)[0x4a090c]
python(PyEval_EvalFrameEx+0x7d2)[0x499a52]
python[0x4a1634]
python(PyRun_FileExFlags+0x92)[0x44e4a5]
python(PyRun_SimpleFileExFlags+0x2ee)[0x44ec9f]
python(Py_Main+0xb5e)[0x44f904]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5)[0x7fcc971a9ec5]
python[0x578c4e]