core: add stat to track unresolved discards in neighbor cache

in __neigh_event_send, if we have a neighbour entry which is in
NUD_INCOMPLETE state, we enqueue any outbound frames to that neighbour
to the neighbours arp_queue, which is default capped to a length of 3
skbs.  If that queue exceeds its set length, it will drop an skb on
the queue to enqueue the newly arrived skb.  This results in a drop
for which we have no statistics incremented.  This patch adds an
unresolved_discards stat to /proc/net/stat/ndisc_cache to track these
lost frames.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -75,7 +75,7 @@ struct neigh_statistics
 	unsigned long destroys;		/* number of destroyed neighs */
 	unsigned long hash_grows;	/* number of hash resizes */
 
-	unsigned long res_failed;	/* nomber of failed resolutions */
+	unsigned long res_failed;	/* number of failed resolutions */
 
 	unsigned long lookups;		/* number of lookups */
 	unsigned long hits;		/* number of hits (among lookups) */
@@ -85,6 +85,8 @@ struct neigh_statistics
 
 	unsigned long periodic_gc_runs;	/* number of periodic GC runs */
 	unsigned long forced_gc_runs;	/* number of forced GC runs */
+
+	unsigned long unres_discards;	/* number of unresolved drops */
 };
 
 #define NEIGH_CACHE_STAT_INC(tbl, field)				\