vulkan: Use fewer rows for scalar FA when HS is not a multiple of 16

[jeffbolznv]

Stacked on #17443.

This FA size came from #17012, and was getting a bunch of register spilling. Perf on RTX 4070 with coopmat1/2 disabled.:

before

FLASH_ATTN_EXT(hsk=72,hsv=72,nh=16,nr23=[1,1],kv=5776,nb=5776,mask=0,sinks=0,max_bias=0.000000,logit_softcap=0.000000,prec=f32,type_KV=f16,permute=[0,1,2,3]):                   6 runs - 167320.33 us/run - 153.73 GFLOP/run - 918.79 GFLOPS

after

FLASH_ATTN_EXT(hsk=72,hsv=72,nh=16,nr23=[1,1],kv=5776,nb=5776,mask=0,sinks=0,max_bias=0.000000,logit_softcap=0.000000,prec=f32,type_KV=f16,permute=[0,1,2,3]):                  34 runs - 30001.44 us/run - 153.73 GFLOP/run -   5.12 TFLOPS