CUDA: fix shared-memory WAR hazard in flash_attn_ext_vec (#30573)
* CUDA: fix KQ write-after-read race in flash_attn_ext_vec
Assisted-by: ChatGPT
* CUDA: apply review suggestions for KQ sync comments
Co-authored-by: Johannes Gäßler <johannesg@5d6.de>
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Co-authored-by: Johannes Gäßler <johannesg@5d6.de>