drbd: cleanup decide_on_discard_support

Sanitize the calling conventions and use a goto label to cleanup the
code flow.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Link: https://lore.kernel.org/r/20220415045258.199825-8-hch@lst.de


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