CUDA: Fix loop unrolling for BW in mul_mat_q_stream_k_fixup (llama/19053)

CUDA: Fix loop unrolling for BW in mul_mat_q_stream_k_fixup (llama/19053)

By providing stride_* variables as size_t (i.e., 64-bit) the compiler can
correctly unroll the [two for-loops](https://github.com/ggml-org/llama.cpp/blob/557515be1e93ed8939dd8a7c7d08765fdbe8be31/ggml/src/ggml-cuda/mmq.cuh#L3789-L3816)
on BW. This gives some perf for prefill/pp phase on BW, while not affecting
other SMs:

| GPU                                                     | Model                 | Test   |   t/s master |   t/s osimons/fix_bw_mmq_fixup_kernel |   Speedup |
|:--------------------------------------------------------|:----------------------|:-------|-------------:|--------------------------------------:|----------:|
| NVIDIA RTX 6000 Ada Generation                          | gpt-oss 20B MXFP4 MoE | pp8096 |      8404.05 |                               8375.79 |      1.00 |
| NVIDIA RTX 6000 Ada Generation                          | llama 3B Q4_K_M       | pp8096 |     16148.93 |                              16019.60 |      0.99 |
| NVIDIA RTX 6000 Ada Generation                          | llama 8B Q4_0         | pp8096 |      8008.29 |                               7978.80 |      1.00 |
| NVIDIA RTX 6000 Ada Generation                          | nemotron_h 9B BF16    | pp8096 |      4263.16 |                               4248.53 |      1.00 |
| NVIDIA RTX 6000 Ada Generation                          | nemotron_h 9B Q4_K_M  | pp8096 |      5165.11 |                               5157.43 |      1.00 |
| NVIDIA RTX PRO 6000 Blackwell Max-Q Workstation Edition | gpt-oss 20B MXFP4 MoE | pp8096 |     12582.80 |                              12758.37 |      1.01 |
| NVIDIA RTX PRO 6000 Blackwell Max-Q Workstation Edition | llama 3B Q4_K_M       | pp8096 |     16879.10 |                              17619.47 |      1.04 |
| NVIDIA RTX PRO 6000 Blackwell Max-Q Workstation Edition | llama 8B Q4_0         | pp8096 |     10649.90 |                              10982.65 |      1.03 |
| NVIDIA RTX PRO 6000 Blackwell Max-Q Workstation Edition | nemotron_h 9B BF16    | pp8096 |      7717.73 |                               7716.22 |      1.00 |
| NVIDIA RTX PRO 6000 Blackwell Max-Q Workstation Edition | nemotron_h 9B Q4_K_M  | pp8096 |      7301.90 |                               7370.38 |      1.01 |