block: separate priority boosting from REQ_META

Add a new REQ_PRIO to let requests preempt others in the cfq I/O schedule,
and lave REQ_META purely for marking requests as metadata in blktrace.

All existing callers of REQ_META except for XFS are updated to also
set REQ_PRIO for now.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Namhyung Kim <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
11 files changed