[IPSEC]: Move ip_summed zapping out of xfrm6_rcv_spi

Not every transform needs to zap ip_summed.  For example, a pure tunnel
mode encapsulation does not affect the hardware checksum at all.  In fact,
every algorithm (that needs this) other than AH6 already does its own
ip_summed zapping.

This patch moves the zapping into AH6 which is in line with what IPv4 does.

Possible future optimisation: Checksum the data as we copy them in IPComp.

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