skb_padto and small fragmented transmits

[Chris Leech <christopher.leech@intel.com>]

While looking at the new software padding routines, something caught my
eye in skb_padto.  It seemed that the fragmented portion of a packet
would actually be counted twice when checking to see if padding is
needed, as skb->len already includes the count of skb->data_len.

> unsigned int size = skb->len + skb->data_len;

I tested this by modifying e1000 to use skb_padto, disabling TCP
timestamps, and writing a small app to transmit 4 bytes using sendfile. 
The resulting packet had 54 bytes of headers, and 4 bytes of data in a
separate fragment.  Calling skb_padto(skb,60) should have linearized the
skb, and zeroed out the first 2 bytes of tailroom.  Instead the length
was incorrectly calculated as 62 bytes, and the buffer was returned as
is.

Changing skb_padto to simply use size = skb->len fixed the padding, but
then I started seeing incorrect TCP checksums going out.  I found this
comment in skb_copy_expand that seemed to explain things.

> BUG ALERT: ip_summed is not copied. Why does this work? Is it used
> only by netfilter in the cases when checksum is recalculated? --ANK

So after calling skb_copy_expand the checksum is not recalculated in
software, but the checksum offload information is discarded.

-- 
Chris Leech <christopher.leech@intel.com>