block: ROW scheduler v4

The ROW I/O scheduler gives priority to READ requests over the WRITE requests when dispatching, without starving WRITE requests. Requests are kept in priority queues. Dispatching is done in a RR manner when the dispatch quantum for each queue is calculated according to queue priority.

Most suitable for mobile devices.

Source : https://patchwork.kernel.org/patch/1862271/

Signed-off-by: andip71 <andreasp@gmx.de>