cfq-iosched: Rename "service_tree" to "st" at some places

At quite a few places we use the keyword "service_tree". At some places,
especially local variables, I have abbreviated it to "st".

Also at couple of places moved binary operator "+" from beginning of line
to end of previous line, as per Tejun's feedback.

v2:
 Reverted most of the service tree name change based on Jeff Moyer's feedback.

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