staging: erofs: introduce generic decompression backend

This patch adds a new generic decompression framework
in order to replace the old LZ4-specific decompression code.

Even though LZ4 is still the only supported algorithm, yet
it is more cleaner and easy to integrate new algorithm than
the old almost hard-coded decompression backend.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>