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: Seongmin Park <r_data@naver.com>

Conflicts:

	block/row-iosched.c

Conflicts:
	block/row-iosched.c

Conflicts:
	block/row-iosched.c
Revert "block: row: Dispatch requests according to their io-priority"

This reverts commit edcfa7523a9c9ffcaf0a982d5ebf497f6a786201.