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.

Change-Id: I5789fb5bd489967bbc393240d27fc0870d893e89
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>
Git-commit: a1fe33af5f8c5f5aebe68d22125d19e48b8e601c
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>