Fix overflow in preprocess_data_kernel of NN Descent (rapidsai#1596)

`preprocess_data_kernel` in NN Descent had overflow issues.
casting `blockIdx.x` to `size_t` to avoid overflow.

Authors:
  - Jinsol Park (https://github.com/jinsolp)

Approvers:
  - Divye Gala (https://github.com/divyegala)

URL: rapidsai#1596

Author: jinsolp

cpp/src/neighbors/detail/nn_descent.cuh

@@ -243,7 +243,11 @@ RAFT_KERNEL preprocess_data_kernel(
   Data_t* s_vec  = (Data_t*)buffer;
   size_t list_id = list_offset + blockIdx.x;
 
-  load_vec(s_vec, input_data + blockIdx.x * dim, dim, dim, threadIdx.x % raft::warp_size());
+  load_vec(s_vec,
+           input_data + static_cast<size_t>(blockIdx.x) * dim,
+           dim,
+           dim,
+           threadIdx.x % raft::warp_size());
   if (threadIdx.x == 0) { l2_norm = 0; }
   __syncthreads();