ublk: return unsigned from ublk_{,un}map_io()

ublk: return unsigned from ublk_{,un}map_io()

ublk_map_io() and ublk_unmap_io() never return negative values, and
their return values are stored in variables of type unsigned. Clarify
that they can't fail by making their return types unsigned.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>