f2fs: compress: add .{init,destroy}_decompress_ctx callback

Add below two callback interfaces in struct f2fs_compress_ops:

	int (*init_decompress_ctx)(struct decompress_io_ctx *dic);
	void (*destroy_decompress_ctx)(struct decompress_io_ctx *dic);

Which will be used by zstd compress algorithm later.

In addition, this patch adds callback function pointer check, so that
specified algorithm can avoid defining unneeded functions.

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