The Xen checksum offload feature attempts to insert a TCP/UDP

The Xen checksum offload feature attempts to insert a TCP/UDP
checksums into already encrypted packets (esp4) in dom0.  Obviously,
it is not possible to insert a checksum into an already encrypted
packet, so this patch inserts the checksum prior to encrypting
packets in net/ipv4/xfrm4_output.c.

To do this cleanly, the TCP/UDP header pointers need to be pointed to
the correct spot, so this functionality has been abstracted into a new
function.

This patch fixes bug 143 (verified by Jim Dykman).  Earlier version
verified by Jon McCune.

Signed-off-by: James Dykman <dykman@us.ibm.com>
Signed-off-by: Jon Mason <jdmason@us.ibm.com>