f2fs: fix error return code in init_f2fs_fs()

Fix to return -ENOMEM in the kset create and add error handling
case instead of 0, as done elsewhere in this function.

Introduced by commit b59d0bae6ca30c496f298881616258f9cde0d9c6.
(f2fs: add sysfs support for controlling the gc_thread)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Namjae Jeon <namjae.jeon@samsung.com>
[Jaegeuk Kim: merge the patch with previous modification]
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>