vulkan: use aligned loads for flash attention mask

[jeffbolznv]

Rewrite the stride logic for the mask tensor in the FA shader to force the stride to be aligned, to allow using more efficient loads.

before:

Z:\github\jeffbolznv\llama.cpp\build\bin\RelWithDebInfo>llama-bench.exe -m C:\models\meta-llama-3-8b-instruct.Q4_K_M.gguf -p 4096,8192,16384 -n 0 -fa 1 --repetitions 1
ggml_vulkan: Found 1 Vulkan devices:
ggml_vulkan: 0 = NVIDIA GeForce RTX 4070 (NVIDIA) | uma: 0 | fp16: 1 | warp size: 32 | shared memory: 49152 | int dot: 1 | matrix cores: NV_coopmat2
| model                          |       size |     params | backend    | ngl | fa |          test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -: | ------------: | -------------------: |
| llama 8B Q4_K - Medium         |   4.58 GiB |     8.03 B | Vulkan     |  99 |  1 |        pp4096 |       3340.34 ± 0.00 |
| llama 8B Q4_K - Medium         |   4.58 GiB |     8.03 B | Vulkan     |  99 |  1 |        pp8192 |       3075.69 ± 0.00 |
| llama 8B Q4_K - Medium         |   4.58 GiB |     8.03 B | Vulkan     |  99 |  1 |       pp16384 |       2599.02 ± 0.00 |

after:

Z:\github\jeffbolznv\llama.cpp\build\bin\RelWithDebInfo>llama-bench.exe -m C:\models\meta-llama-3-8b-instruct.Q4_K_M.gguf -p 4096,8192,16384 -n 0 -fa 1 --repetitions 1
ggml_vulkan: Found 1 Vulkan devices:
ggml_vulkan: 0 = NVIDIA GeForce RTX 4070 (NVIDIA) | uma: 0 | fp16: 1 | warp size: 32 | shared memory: 49152 | int dot: 1 | matrix cores: NV_coopmat2
| model                          |       size |     params | backend    | ngl | fa |          test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -: | ------------: | -------------------: |
| llama 8B Q4_K - Medium         |   4.58 GiB |     8.03 B | Vulkan     |  99 |  1 |        pp4096 |       3378.30 ± 0.00 |
| llama 8B Q4_K - Medium         |   4.58 GiB |     8.03 B | Vulkan     |  99 |  1 |        pp8192 |       3093.57 ± 0.00 |
| llama 8B Q4_K - Medium         |   4.58 GiB |     8.03 B | Vulkan     |  99 |  1 |       pp16384 |       2681.14 ± 0.00 |