s390/qeth: handle skb allocation error gracefully

When current code fails to allocate an skb in the RX path, it drops the
whole RX buffer. Considering the large number of packets that a single
RX buffer might contain, this is quite drastic.

Skip over the packet instead, and try to extract the next packet from
the RX buffer.

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