skbuff: skb_mac_header_was_set is always true on >32 bit

Looking at the crash in log_martians(), one suspect is that the check for
mac header being set is not correct.  The value of mac_header defaults to
0 on allocation, therefore skb_mac_header_was_set will always be true on
platforms using NET_SKBUFF_USES_OFFSET.
Signed-off-by: NStephen Hemminger <shemminger@vyatta.com>
Acked-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -201,6 +201,10 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 	skb->data = data;
 	skb_reset_tail_pointer(skb);
 	skb->end = skb->tail + size;
+#ifdef NET_SKBUFF_DATA_USES_OFFSET
+	skb->mac_header = ~0U;
+#endif
+
 	/* make sure we initialize shinfo sequentially */
 	shinfo = skb_shinfo(skb);
 	atomic_set(&shinfo->dataref, 1);