block: Do not wake the request CPU if idle

Refrain from waking up an idle CPU if possible since the exit latency
of taking req->cpu out of an idle cstate will likely exceed the
rq->deadline constraint compared to executing the request locally
instead.

Signed-off-by: Tyler Nijmeh <tylernij@gmail.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>