selinux: add support for RTM_NEWCHAIN, RTM_DELCHAIN, and RTM_GETCHAIN

Commit 32a4f5ec ("net: sched: introduce chain object to uapi")
added new RTM_* definitions without properly updating SELinux, this
patch adds the necessary SELinux support.

While there was a BUILD_BUG_ON() in the SELinux code to protect from
exactly this case, it was bypassed in the broken commit.  In order to
hopefully prevent this from happening in the future, add additional
comments which provide some instructions on how to resolve the
BUILD_BUG_ON() failures.

Fixes: 32a4f5ec ("net: sched: introduce chain object to uapi")
Cc: <stable@vger.kernel.org> # 4.19
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Moore <paul@paul-moore.com>