f2fs: constify super_operations

super_operations are not supposed to change at runtime.
"struct super_block" working with super_operations provided
by <linux/fs.h> work with const super_operations. So mark
the non-const structs as const

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>