fs: dlm: fix return value check in dlm_memory_init()

It should check 'cb_cache', after calling kmem_cache_create("dlm_cb").

Fixes: 61bed0ba ("fs: dlm: use a non-static queue for callbacks")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David Teigland <teigland@redhat.com>