block: add ROW scheduler v4

This patch adds the implementation of a new scheduling algorithm - ROW.
The policy of this algorithm is to prioritize READ requests over WRITE
as much as possible without starving the WRITE requests.
The requests are kept in queues according to their priority. The dispatch
is done in a Round Robin manner with a different slice for each queue.
READ request queues get bigger dispatch quantum than the write requests.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>

Conflicts:
	block/Kconfig.iosched
	block/Makefile

Signed-off-by: Humberto Borba <humberos@gmail.com>
row: Adding support for reinsert already dispatched req

Add support for reinserting already dispatched request back to the
schedulers internal data structures.
The request will be reinserted back to the queue (head) it was
dispatched from as if it was never dispatched.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
row: Add support for urgent request handling

This patch add support for handling urgent requests.
ROW queue can be marked as "urgent". If an urgent queue was
un-served in a previous dispatch cycle and a request was added
to it - it will trigger issuing urgent request to the device driver.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>