In net/http package, the ServeContent/ServeFile doesn't check the I/O
timeout error from chunkWriter or *net.TCPConn, which means that both
HTTP status and headers might be missing when WriteTimeout happens. If
the poll.SendFile() doesn't check the *poll.FD state before sending
data, the client will only receive the response body with status and
report "malformed http response/status code".
This patch is to enable netpollcheckerr before sendfile, which should
align with normal *poll.FD.Write() and Splice().