[XFRM] user: Move attribute copying code into copy_to_user_state_extra

Here's a good example of code duplication leading to code rot.  The
notification patch did its own netlink message creation for xfrm states.
It duplicated code that was already in dump_one_state.  Guess what, the
next time (and the time after) when someone updated dump_one_state the
notification path got zilch.

This patch moves that code from dump_one_state to copy_to_user_state_extra
and uses it in xfrm_notify_sa too.  Unfortunately whoever updates this
still needs to update xfrm_sa_len since the notification path wants to
know the exact size for allocation.

At least I've added a comment saying so and if someone still forgest, we'll
have a WARN_ON telling us so.

I also changed the security size calculation to use xfrm_user_sec_ctx since
that's what we actually put into the skb.  However it makes no practical
difference since it has the same size as xfrm_sec_ctx.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>