xen-blkfront: rely on the default discard granularity

The block layer now sets the discard granularity to the physical
block size default.  Take advantage of that in xen-blkfront and only
set the discard granularity if explicitly specified.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20240221125845.3610668-3-hch@lst.de


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