ipsec: Use RCU-like construct for saved state within a walk

Now that we save states within a walk we need synchronisation
so that the list the saved state is on doesn't disappear from
under us.

As it stands this is done by keeping the state on the list which
is bad because it gets in the way of the management of the state
life-cycle.

An alternative is to make our own pseudo-RCU system where we use
counters to indicate which state can't be freed immediately as
it may be referenced by an ongoing walk when that resumes.

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