SELinux: Write class field in role_trans_write.

If kernel policy version is >= 26, then write the class field of the
role_trans structure into the binary reprensentation.

Signed-off-by: Harry Ciao <qingtao.cao@windriver.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Eric Paris <eparis@redhat.com>