s390/qeth: check size of required HW header cache object

When qeth_add_hw_header() falls back to the header cache, ensure that
the requested length doesn't exceed the object size.

For current usage this is a no-brainer, but TSO transmission will
introduce protocol headers of varying length.

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