nvmet: ignore EOPNOTSUPP for discard

NVMe DSM is a pure hint, so if the underlying device / file system
does not support discard-like operations we should not fail the
operation but rather return success.

Fixes: 3b031d15

 ("nvmet: add error log support for bdev backend")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Tested-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>