SELinux: seperate range transition rules to a seperate function

Move the range transition rule to a separate function, range_read(), rather
than doing it all in policydb_read()

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>