sd: add a sd_disable_discard helper

sd: add a sd_disable_discard helper

Add helper to disable discard when it is not supported and use it
instead of sd_config_discard in the I/O completion handler.  This avoids
touching more fields than required in the I/O completion handler and
prepares for converting sd to use the atomic queue limits API.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>