f2fs: avoid naming confusion of sysfs init

This patch changes the function names of sysfs init to follow ext4.

f2fs_init_sysfs <-> f2fs_register_sysfs
f2fs_exit_sysfs <-> f2fs_unregister_sysfs

Suggested-by: Chao Yu <yuchao0@huawei.com>
Reivewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>