cfq-iosched: Properly name all references to IO class

Currently CFQ has three IO classes, RT, BE and IDLE. At many a places we
are calling workloads belonging to these classes as "prio". This gets
very confusing as one starts to associate it with ioprio.

So this patch just does bunch of renaming so that reading code becomes
easier. All reference to RT, BE and IDLE workload are done using keyword
"class" and all references to subclass, SYNC, SYNC-IDLE, ASYNC are made
using keyword "type".

This makes me feel much better while I am reading the code. There is no
functionality change due to this patch.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>