xz: add fall-through comments to a switch statement

xz: add fall-through comments to a switch statement

It's good style. I was also told that GCC 7 is more strict and might
give a warning when such comments are missing.

Suggested-by: Andrei Borzenkov <arvidjaar@gmail.com>
Signed-off-by: Lasse Collin <lasse.collin@tukaani.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Origin: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 5a244f48ecbb
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>