xen: netfront: handle incoming GSO SKBs which are not CHECKSUM_PARTIAL

xen: netfront: handle incoming GSO SKBs which are not CHECKSUM_PARTIAL

The Linux network stack expects all GSO SKBs to have ip_summed ==
CHECKSUM_PARTIAL (which implies that the frame contains a partial
checksum) and the Xen network ring protocol similarly expects an SKB
which has GSO set to also have NETRX_csum_blank (which also implies a
partial checksum).

However there have been cases of buggy guests which mark a frame as
GSO but do not set csum_blank. If we detect that we a receiving such a
frame (which manifests as ip_summed != PARTIAL && skb_is_gso) then
force the SKB to partial and recalculate the checksum, since we cannot
rely on the peer having done so if they have not set csum_blank.

Add an ethtool stat to track occurances of this event.

Signed-off-by: Ian Campbell <[email protected]>
Cc: Jeremy Fitzhardinge <[email protected]>
Cc: David Miller <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: David S. Miller <[email protected]>