Don't build discards bigger than what the user asked for, if the
user decided to limit the size by writing to 'discard_max_bytes'.
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>