ixgbe: skip non IPv4 packets in ATR filter

In driver ixgbe, ixgbe_atr may cause crashes for non-ipv4 packets. Just
add a test to check skb->protocol.  It may crash on short packets due
to ip_hdr() access.

Signed-off-by: Guillaume Gaudonville <guillaume.gaudonville@6wind.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>