s390/qeth: pass TSO data offset to fill_buffer()

For TSO we need to skip the skb's qeth/IP/TCP headers when mapping
it into buffer elements. Instead of (mis)using skb_pull(), pass a
corresponding offset to fill_buffer() like we already do for IQDs.

No actual change in the resulting TSO buffers.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>