block: row: Dispatch requests according to their io-priority

This patch implements "application-hints" which is a way the issuing
application can notify the scheduler on the priority of its request.
This is done by setting the io-priority of the request.
This patch reuses an already existing mechanism of io-priorities developed
for CFQ. Please refer to kernel/Documentation/block/ioprio.txt for
usage example and explanations.

Change-Id: I228ec8e52161b424242bb7bb133418dc8b73925a
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>

Conflicts:
	block/row-iosched.c

+ ...
block: row: Idling mechanism re-factoring

At the moment idling in ROW is implemented by delayed work that uses
jiffies granularity which is not very accurate. This patch replaces
current idling mechanism implementation with hrtime API, which gives
nanosecond resolution (instead of jiffies).

Change-Id: I86c7b1776d035e1d81571894b300228c8b8f2d92
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>