erofs: introduce struct z_erofs_decompress_backend

Let's introduce struct z_erofs_decompress_backend in order to pass
on the decompression backend context between helper functions more
easier and avoid too many arguments.

Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220715154203.48093-13-hsiangkao@linux.alibaba.com