io_uring: support vectored kernel fixed buffer

io_uring has supported fixed kernel buffer via io_buffer_register_bvec()
and io_buffer_unregister_bvec().

The vectored fixed buffer has been ready, so it is natural to support
fixed kernel buffer, one use case is ublk.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250325135155.935398-4-ming.lei@redhat.com


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