Update iocontext.h

Adding code for IOPS

"struct cfq_io_context" should be cleaned. New definition of struct cfq_io_context:
struct cfq_ttime {
        unsigned long last_end_request;
        unsigned long ttime_total;
        unsigned long ttime_samples;
        unsigned long ttime_mean;
};

struct cfq_io_context {
        struct dev_io_context dev_ioc;
        struct cfq_queue *cfqq[2];
        struct cfq_ttime ttime;
};