[IPSEC]: Make x->lastused an unsigned long

Currently x->lastused is u64 which means that it cannot be
read/written atomically on all architectures.  David Miller observed
that the value stored in it is only an unsigned long which is always
atomic.

So based on his suggestion this patch changes the internal
representation from u64 to unsigned long while the user-interface
still refers to it as u64.

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