sd: simplify the ZBC case in provisioning_mode_store

sd: simplify the ZBC case in provisioning_mode_store

Don't reset the discard settings to no-op over and over when a user
writes to the provisioning attribute as that is already the default
mode for ZBC devices.  In hindsight we should have made writing to
the attribute fail for ZBC devices, but the code has probably been
around for far too long to change this now.

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>