gfs2: Fix wrong error handling in init_gfs2_fs()

init_gfs2_fs() is calling e.g. calling unregister_shrinker() without
register_shrinker() when an error occurred during initialization.
Rename goto labels and call appropriate undo function.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>