SELinux: move genfs read to a separate function

move genfs read functionality out of policydb_read() and into a new
function called genfs_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>