s390/qeth: ensure linear access to packet headers

When the RX path builds non-linear skbs, the packet headers can
currently spill over into page fragments. Depending on the packet type
and what fields we need to access in the headers, this could cause us
to go past the end of skb->data.

So for non-linear packets, copy precisely the length of the necessary
headers ('linear_len') into skb->data.
And don't copy more, upper-level protocols will peel whatever additional
packet headers they need.

Fixes: 4a71df50

 ("qeth: new qeth device driver")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>