SELinux: break ocontext reading into a separate function

Move the reading of ocontext type data out of policydb_read() in a separate
function ocontext_read()

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