block: do not notify urgent request, when flush with data in flight

MMC device driver implements URGENT request execution with priority
(using stop flow), as a result currently running (and prepeared) request
may be reinserted back into I/O scheduler. This will break block layer
logic of flushes (flush request should not be inserted into I/O scheduler).

Block layer flush machinery keep q->flush_data_in_flight list updated with
started but not completed flush requests with data (REQ_FUA).

This change will not notify underling block device driver about pending
urgent request during flushes in flight.

Change-Id: I5f7bb23583ac755fe76d6123b99315f2ba49fac2
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>