SELinux: define audit permissions for audit tree netlink messages

Audit trees defined 2 new netlink messages but the netlink mapping tables for
selinux permissions were not set up.  This patch maps these 2 new operations
to AUDIT_WRITE.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>