ublk_drv: fix error handling of ublk_add_dev

ublk_drv: fix error handling of ublk_add_dev

__ublk_destroy_dev() is called for handling error in ublk_add_dev(),
but either tagset isn't allocated or mutex isn't initialized.

So fix the issue by letting replacing ublk_add_dev with a
ublk_add_tag_set function that is much more limited in scope and
instead unwind every single step directly in ublk_ctrl_add_dev.
To allow for this refactor the device freeing so that there is
a helper for freeing the device number instead of coupling that
with freeing the mutex and the memory.

Note that this now copies the dev_info to userspace before adding
the character device.  This not only simplifies the erro handling
in ublk_ctrl_add_dev, but also means that the character device
can only be seen by userspace if the device addition succeeded.

Based on a patch from Ming Lei.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220722103817.631258-2-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>