Squashfs: add a decompressor framework

This adds a decompressor framework which allows multiple compression
algorithms to be cleanly supported.

Also update zlib wrapper and other code to use the new framework.

Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>

diff --cc fs/squashfs/Makefile
Simple merge

diff --cc fs/squashfs/block.c
Simple merge

diff --cc fs/squashfs/decompressor.c
Simple merge

diff --cc fs/squashfs/decompressor.h
Simple merge

diff --cc fs/squashfs/squashfs.h
Simple merge

diff --cc fs/squashfs/squashfs_fs_sb.h
Simple merge

diff --cc fs/squashfs/super.c
Simple merge

diff --cc fs/squashfs/zlib_wrapper.c
Simple merge