sd: retry discard commands

Currently cmd->allowed is initialized from rq->retries for discard
commands, but retries is always 0 for non-BLOCK_PC requests.  Set it
to the standard number of retries instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>