io_uring/net: reuse req->buf_index for sendzc

io_uring/net: reuse req->buf_index for sendzc

There is already a field in io_kiocb that can store a registered buffer
index, use that instead of stashing the value into struct io_sr_msg.

Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250224213116.3509093-4-kbusch@meta.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>