flash: Reduce smem_scratchpad alloc size

tests/regression/flash_attention/kernel.cpp

@@ -545,7 +545,7 @@ void kernel_body(int task_id, kernel_arg_t *__UNIFORM__ arg) {
   // NOTE: out-of bounds is not checked
   // TODO: reduce this from B_ROW to NUM_WARPS
   constexpr uint32_t smem_scratchpad_size =
-      B_ROW * NUM_THREADS * 2 /*arbitrary slack*/;
+      threads_per_warpgroup * 2 /*arbitrary slack*/;
   float *smem_scratchpad = smem_O_row_scale_1 - smem_scratchpad_size;
 
   // initialize rowmax/rowsum values in sharedmem