[XFRM]: Export SAD info.

On a system with a lot of SAs, counting SAD entries chews useful
CPU time since you need to dump the whole SAD to user space;
i.e something like ip xfrm state ls | grep -i src | wc -l
I have seen taking literally minutes on a 40K SAs when the system
is swapping.
With this patch, some of the SAD info (that was already being tracked)
is exposed to user space. i.e you do:
ip xfrm state count
And you get the count; you can also pass -s to the command line and
get the hash info.

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>