fix interrupt lost bug when call add_event_detect
do'nt set EPOLLET ,if EPOLLET is set , after epoll_wait, we must read until return EAGAIN,but maybe driver will not return EAGAIN,the better choice is do'nt use this current epoll_wait cause interrupt lost
fix a serious bug ,the buffer may visit after it is free,when the buffered size is more than "mbuf_thresh" in mbuf.c