It's not needed as the REQ_F_BUFFER_SELECTED flag tracks the state of
whether or not kbuf is valid, so just drop it.
Suggested-by: Dylan Yudaken <dylany@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
 {
        req->flags &= ~REQ_F_BUFFER_SELECTED;
        list_add(&req->kbuf->list, list);
-       req->kbuf = NULL;
 
        return IORING_CQE_F_BUFFER | (req->buf_index << IORING_CQE_BUFFER_SHIFT);
 }
        list_add(&buf->list, &bl->buf_list);
        req->flags &= ~REQ_F_BUFFER_SELECTED;
        req->buf_index = buf->bgid;
-       req->kbuf = NULL;
 
        io_ring_submit_unlock(ctx, issue_flags);
 }