There is a newly introduced hrtimer_setup() which will replace
hrtimer_init(). This new function is similar to the old one, except that it
also sanity-checks and initializes the timer's callback function.
Switch to use the new function.
This new function is also used to initialize the callback function in
.prep() (the callback function depends on whether it is IORING_OP_TIMEOUT
or IORING_OP_LINK_TIMEOUT). Thus, callback function setup in io_timeout()
and io_queue_linked_timeout() are now redundant, therefore remove them.