ublk: zc register/unregister bvec

Provide new operations for the user to request mapping an active request
to an io uring instance's buf_table. The user has to provide the index
it wants to install the buffer.

A reference count is taken on the request to ensure it can't be
completed while it is active in a ring's buf_table.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20250227223916.143006-6-kbusch@meta.com


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