ublk: add helper to check if device supports user copy

This will be used by ublk zoned storage support.

Signed-off-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20230804114610.179530-2-nmi@metaspace.dk


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