CUDA: Fix ssm_scan_f32 data-races (#24360)

• Add missing syncthreads before resuing cub_temp_storage

__syncthreads() is required before being allowed to resue TempStorage smem: https://nvidia.github.io/cccl/unstable/cub/api/classcub_1_1BlockLoad.html#_CPPv4I0EN3cub9BlockLoad4LoadEv20RandomAccessIteratorRA14ItemsPerThread_1Ti

• Add one more missing __syncthreads

Could also double-buffer, but alternative is to simply ensure all threads have read smem* before writing to it again in the next loop iteration

• Remove unused smem from ssm_scan_f32