sbitmap: warn if using smaller shallow depth than was setup

Make sure the user passed the right value to
sbitmap_queue_min_shallow_depth().

Acked-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>