Fix a wrong reference to seqlen_k variable in the varlen kernel

[cakeng]

This fixes a line in compute_attn_1rowblock_splitkv kernel function that uses params.seqlen_k to use binfo.actual_seqlen_k instead to access the seqlen_k value.

This is a fix to the flash_attn_varlen_func function producing wrong results when used with cudaGraph over different input parameters, as found in the vLLM repo (vllm-project/vllm#6052).