io_uring: combine buffer lookup and import

io_uring: combine buffer lookup and import

Registered buffer are currently imported in two steps, first we lookup
a rsrc node and then use it to set up the iterator. The first part is
usually done at the prep stage, and import happens whenever it's needed.
As we want to defer binding to a node so that it works with linked
requests, combine both steps into a single helper.

Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250224213116.3509093-6-kbusch@meta.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>