ublk_drv: only allow owner to open unprivileged disk

ublk_drv: only allow owner to open unprivileged disk

Owner of one unprivileged ublk device could be one evil user, which
can grant this disk's privilege to other users deliberately, and
this way could be like making one trap and waiting for other users
to be caught.

So only owner to open unprivileged disk even though the owner
grants disk privilege to other user. This way is reasonable too
given anyone can create ublk disk, and no need other's grant.

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Fixes: 4093cb5a0634 ("ublk_drv: add mechanism for supporting unprivileged ublk device")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230131040446.214583-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>