ublk: Make ublks_max configurable

We are converting tcmu applications to ublk, but have systems with up
to 1k devices. This patch allows us to configure the ublks_max from
userspace with the ublks_max modparam.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20231012150600.6198-3-michael.christie@oracle.com


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