blk-wbt: throttle discards like background writes

Throttle discards like we would any background write. Discards should
be background activity, so if they are impacting foreground IO, then
we will throttle them down.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>