mksquashfs: shorten the {reader|fragment|writer}_buffer_size variables

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
mksquashfs: fix a potential non-default option deadlock

Fix a potential deadlock in Mksquashfs that may be triggerable using
non-default options.

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