blk-wbt: add wbt_any_inflight()

blk-wbt: add wbt_any_inflight()

Some callsites care about the specific number of writes inflight,
others only care if any writes are inflight or not. In preparation
for increasing the number of wait buckets, add wbt_any_inflight()
to avoid iterating and checking more buckets than we need.

Signed-off-by: Jens Axboe <axboe@kernel.dk>