block, cfq: restructure io_cq creation path for io_context interface cleanup

Add elevator_ops->elevator_init_icq_fn() and restructure
cfq_create_cic() and rename it to ioc_create_icq().

The new function expects its caller to pass in io_context, uses
elevator_type->icq_cache, handles generic init, calls the new elevator
operation for elevator specific initialization, and returns pointer to
created or looked up icq.  This leaves cfq_icq_pool variable without
any user.  Removed.

This prepares for io_context interface cleanup and doesn't introduce
any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>