metal : adjust extra size for FA buffer to avoid reallocations (#18545)

ggml/src/ggml-metal/ggml-metal-ops.cpp

@@ -2181,7 +2181,11 @@ size_t ggml_metal_op_flash_attn_ext_extra_pad(const ggml_tensor * op) {
 
     const bool has_mask = op->src[3] != nullptr;
 
-    if (ggml_metal_op_flash_attn_ext_use_vec(op)) {
+    // note: the non-vec kernel requires more extra memory, so always reserve for it
+    GGML_ASSERT(OP_FLASH_ATTN_EXT_NCPSG >= OP_FLASH_ATTN_EXT_VEC_NCPSG);
+
+    //if (ggml_metal_op_flash_attn_ext_use_vec(op)) {
+    if (false) {
         // note: always reserve the padding space to avoid graph reallocations
         //const bool has_kvpad = ne11 % OP_FLASH_ATTN_EXT_VEC_NCPSG != 0;
         const bool has_kvpad = true;