metal : fix QK unroll condition

Author: ggerganov

ggml/src/ggml-metal/ggml-metal.metal

@@ -4791,7 +4791,7 @@ void kernel_flash_attn_ext_impl(
                 for (short cc = 0; cc < NC; ++cc) {
                     qk8x8_t mqk = make_filled_simdgroup_matrix<qk_t, 8>((qk_t) 0.0f);
 
-                    if (DK8 % 16 != 0) {
+                    if (DK % 16 != 0) {
                         k8x8_t mk;
                         q8x8_t mq;