cfq-iosched: Rename few functions related to selecting workload
choose_service_tree() selects/sets both wl_class and wl_type. Rename it to
choose_wl_class_and_type() to make it very clear.
cfq_choose_wl() only selects and sets wl_type. It is easy to confuse
it with choose_st(). So rename it to cfq_choose_wl_type() to make
it clear what does it do.