CUDA: Fix ssm_scan_f32 data-races (llama/24360)

CUDA: Fix ssm_scan_f32 data-races (llama/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