ubifs: fix to check error code of register_shrinker

register_shrinker() in ubifs_init() can fail due to fail to call kzalloc.
This patch fixes to check the return value of register_shrinker, otherwise
our shrinker may be unregistered after ubifs initialized successfully.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>