[IPSEC]: Kill unused decap state argument

This patch removes the decap_state argument from the xfrm input hook.
Previously this function allowed the input hook to share state with
the post_input hook.  The latter has since been removed.

The only purpose for it now is to check the encap type.  However, it
is easier and better to move the encap type check to the generic
xfrm_rcv function.  This allows us to get rid of the decap state
argument altogether.

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