This is another patch on the way towards a unified XFRM tunnel
encapsulation function.

This patch changes the value of props.header_len for IPCOMP to
exclude the IPCOMP header.  The reason is that the IPCOMP header
is added only if the packet is compressible.  That is, if the
size of the compressed payload plus the size of the IPCOMP header
is less than that of the original payload.

This means that the IPCOMP encapsulation does not impose any
overhead at all as far as the MTU is concerned.  The current
code incorrectly reduces the MTU by the size of the IPCOMP
header.

As a side-effect, this means that we don't have to move the
IP header around when IPCOMP is used.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@redhat.com>