jffs2: drop unused model argument

The jffs2 compression framework provides a "model" argument when
compressing and decompressing, but the caller always passes in NULL
and the callees never use it.  So punt this useless overhead.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>