Squashfs: Add LZ4 compression configuration option

Add the glue code, and also update the documentation.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>

diff --cc Documentation/filesystems/squashfs.txt
Simple merge

diff --cc fs/squashfs/Kconfig
Simple merge

diff --cc fs/squashfs/Makefile
Simple merge

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

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