fix race in fusedL2knn smem read/write by adding a syncwarp (#679)

-- fix race in fusedL2knn smem read/write by adding a syncwarp reported by racecheck tool of compute-sanitizer.
-- this addresses issue - #676

Authors:
  - Mahesh Doijade (https://github.com/mdoijade)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #679

cpp/include/raft/spatial/knn/detail/fused_l2_knn.cuh

@@ -446,6 +446,7 @@ __global__ __launch_bounds__(Policy::Nthreads, 2) void fusedL2kNN(const DataT* x
                   }
                 }
               }
+              __syncwarp();
               const int finalNumVals = raft::shfl(numValsWarpTopK[i], 31);
               loadWarpQShmem<Policy, Pair>(heapArr[i], &shDumpKV[0], rowId, numOfNN);
               updateSortedWarpQ<Pair, myWarpSelect::kNumWarpQRegisters>(