balloon: Fix JNI detach in request_memory()

The request_memory() function has an early exit condition where we
forgot to call _detach().  Fix that up by attaching later in the
function which ensures we eventually will detach.

Signed-off-by: wangbicheng <wangbicheng@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>