selinux: fix type mismatch

avc_cache_threshold is of type unsigned int.  Do not use a signed
new_value in sscanf(page, "%u", &new_value).

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
[PM: subject prefix fix, description cleanup]
Signed-off-by: Paul Moore <paul@paul-moore.com>