net: add skb_checksum_setup

net: add skb_checksum_setup

This patch adds a function to set up the partial checksum offset for IP
packets (and optionally re-calculate the pseudo-header checksum) into the
core network code.
The implementation was previously private and duplicated between xen-netback
and xen-netfront, however it is not xen-specific and is potentially useful
to any network driver.

Signed-off-by: Paul Durrant <[email protected]>
Cc: David Miller <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Veaceslav Falico <[email protected]>
Cc: Alexander Duyck <[email protected]>
Cc: Nicolas Dichtel <[email protected]>
Signed-off-by: David S. Miller <[email protected]>