block: add REQ_URGENT to request flags

This patch adds a new flag to be used in cmd_flags field of struct request
for marking request as urgent.
Urgent request is the one that should be given priority currently handled
(regular) request by the device driver. The decision of a request urgency
is taken by the scheduler.

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

Conflicts:

	include/linux/blk_types.h
Revert "block: row-iosched idling triggered by readahead pages"

This reverts commit 200da345a0636f043b9e099b4efd5d34bca34bcb.
block: add sio
block: add fiops io scheduler