s390/qeth: pass full data length to l3_fill_header()

The TSO and IQD paths already need to fix-up the current values, and
OSA will require more flexibility in the future as well. So just let
the caller specify the data length.

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