io_uring/rw: move fixed buffer import to issue path

Registered buffers may depend on a linked command, which makes the prep
path too early to import. Move to the issue path when the node is
actually needed like all the other users of fixed buffers.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20250227223916.143006-3-kbusch@meta.com


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