sbitmap: add helper to inform the core about shallow depth limiting

sbitmap: add helper to inform the core about shallow depth limiting

If a user of sbitmap limits the shallow depth on his own, then
we need to inform the lower layers. Otherwise we can run into
a situation where the user has limited the depth to something
smaller than our wake batch count, which can cause hangs.

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