vulkan : fix FA mask load with bounds check (coopmat2) (#17606)

Author: Acly

ggml/src/ggml-vulkan/vulkan-shaders/flash_attn_cm2.comp

@@ -156,7 +156,7 @@ void main() {
                 tensorLayoutM = setTensorLayoutStrideNV(tensorLayoutM, m_stride, 1);
                 tensorLayoutM = setTensorLayoutClampValueNV(tensorLayoutM, 0xfc00); // -inf in float16_t
 
-                coopmat<float16_t, gl_ScopeWorkgroup, Br, Bc, gl_MatrixUseAccumulator> mv, mvmax;
+                coopmat<float16_t, gl_ScopeWorkgroup, Br, Bc, gl_MatrixUseAccumulator> mvmax;
 
                 coopMatLoadTensorNV(mv, data_m, m_offset, sliceTensorLayoutNV(tensorLayoutM, i * Br, Br, j * Bc, Bc));