[JFFS2] jffs2_acl_count() tests < 0 on unsigned

size_t s is unsigned and cannot be less than 0.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>