gfs2: fix to detect failure of register_shrinker

register_shrinker can fail after commit 1d3d4437

 ("vmscan: per-node
deferred work"), we should detect the failure of it, otherwise we may
fail to register shrinker after gfs2 module was been inited successfully.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>