[IPSEC]: Fix transport-mode async resume on intput without netfilter

When netfilter is off the transport-mode async resumption doesn't work
because we don't push back the IP header.  This patch fixes that by
moving most of the code outside of ifdef NETFILTER since the only part
that's not common is the short-circuit in the protocol handler.

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