net: Embed hh_cache inside of struct neighbour.

Now that there is a one-to-one correspondance between neighbour
and hh_cache entries, we no longer need:

1) dynamic allocation
2) attachment to dst->hh
3) refcounting

Initialization of the hh_cache entry is indicated by hh_len
being non-zero, and such initialization is always done with
the neighbour's lock held as a writer.

Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -426,9 +426,10 @@ static int rt_cache_seq_show(struct seq_file *seq, void *v)
 			(int)((dst_metric(&r->dst, RTAX_RTT) >> 3) +
 			      dst_metric(&r->dst, RTAX_RTTVAR)),
 			r->rt_key_tos,
-			r->dst.hh ? atomic_read(&r->dst.hh->hh_refcnt) : -1,
-			r->dst.hh ? (r->dst.hh->hh_output ==
-				       dev_queue_xmit) : 0,
+			-1,
+			(r->dst.neighbour ?
+			 (r->dst.neighbour->hh.hh_output ==
+			  dev_queue_xmit) : 0),
 			r->rt_spec_dst, &len);
 
 		seq_printf(seq, "%*s\n", 127 - len, "");