net: Loosen constraints for recalculating checksum in skb_segment()

commit 1cdbcb7957cf9e5f841dbcde9b38fd18a804208b
Author: Simon Horman [off-list ref]
Date:   Sun May 19 15:46:49 2013 +0000

net: Loosen constraints for recalculating checksum in skb_segment()

This is a generic solution to resolve a specific problem that I have
observed.

If the encapsulation of an skb changes then ability to offload
checksums may also change. In particular it may be necessary to
perform checksumming in software.

An example of such a case is where a non-GRE packet is received but
is to be encapsulated and transmitted as GRE.

Dave, can you queue this for 3.10-stable.

It was around 3.9 when ip_gre got gso support. This added the bug that
forwarding non-gre to gre with gro+gso+tx offloads enabled does not
work properly - the TCP checksums are not calculated properly without
this patch.

Thanks,
 Timo