Fix the bug introduced by da3f56cb (migration: remove
ram_save_compressed_page()), It should be 'return' rather than
'res'

Sorry for this stupid mistake :(
Signed-off-by: NXiao Guangrong <xiaoguangrong@tencent.com>
Message-Id: <20180428081045.8878-1-xiaoguangrong@tencent.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>