ggml-opencl : store GPU buffer in ggml_tensor::extra

[slaren]

ggml-opencl currently stores the GPU buffer in ggml_tensor::data, and after the GGUF changes this will result in a memory leak when not using mmap, as the address of the CPU buffer is lost after the call to ggml_cl_transform_tensor:

llama.cpp/llama.cpp

Lines 1516 to 1523 in 47068e5

	#elif defined(GGML_USE_CLBLAST)
	case GGML_BACKEND_GPU:
	ggml_cl_transform_tensor(cur->data, cur);
	if (!use_mmap) {
	free(cur->data);
	}
	break;
	#endif

This change solves that issue and a possible interaction with ggml-alloc if the opencl buffer falls within the measure buffer memory range by storing the GPU buffer in ggml_tensor::extra instead.

Fixes #2993