[NET]: Force a cache line split in hh_cache in SMP.

hh_lock was converted from rwlock to seqlock by Stephen.

To have a 100% benefit of this change, I suggest to place read mostly fields
of hh_cache in a separate cache line, because hh_refcnt may be changed quite
frequently on some busy machines.
Signed-off-by: NEric Dumazet <dada1@cosmosbay.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -193,7 +193,14 @@ struct hh_cache
 {
 	struct hh_cache *hh_next;	/* Next entry			     */
 	atomic_t	hh_refcnt;	/* number of users                   */
-	__be16		hh_type;	/* protocol identifier, f.e ETH_P_IP
+/*
+ * We want hh_output, hh_len, hh_lock and hh_data be a in a separate
+ * cache line on SMP.
+ * They are mostly read, but hh_refcnt may be changed quite frequently,
+ * incurring cache line ping pongs.
+ */
+	__be16		hh_type ____cacheline_aligned_in_smp;
+					/* protocol identifier, f.e ETH_P_IP
                                          *  NOTE:  For VLANs, this will be the
                                          *  encapuslated type. --BLG
                                          */