xen-netfront: use correct linear area after linearizing an skb

xen-netfront: use correct linear area after linearizing an skb

Commit b21961408509791f50f4879d98b3e1e88a8b7e93 (xen-netfront: Fix
handling packets on compound pages with skb_linearize) attempted to
fix a problem where an skb that would have required too many slots
would be dropped causing TCP connections to stall.

However, it filled in the first slot using the original buffer and not
the new one and would use the wrong offset and grant access to the
wrong page.

Netback would notice the malformed request and stop all traffic on the
VIF, reporting:

    vif vif-3-0 vif3.0: txreq.offset: 85e, size: 4002, end: 6144
    vif vif-3-0 vif3.0: fatal error; disabling device

Reported-by: Anthony Wright <anthony@overnetdata.com>
Tested-by: Anthony Wright <anthony@overnetdata.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>