net: add explicit logging and stat for neighbour table overflow

Add an explicit neighbour table overflow message (ratelimited) and
statistic to make diagnosing neighbour table overflows tractable in
the wild.

Diagnosing a neighbour table overflow can be quite difficult in the wild
because there is no explicit dmesg logged.  Callers to neighbour code
seem to use net_dbg_ratelimit when the neighbour call fails which means
the "base message" is not emitted and the callback suppressed messages
from the ratelimiting can end-up juxtaposed with unrelated messages.
Further, a forced garbage collection will increment a stat on each call
whether it was successful in freeing-up a table entry or not, so that
statistic is only a hint.  So, add a net_info_ratelimited message and
explicit statistic to the neighbour code.

Signed-off-by: Rick Jones <rick.jones2@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/neighbour.h

@@ -106,6 +106,7 @@ struct ndt_stats {
 	__u64		ndts_rcv_probes_ucast;
 	__u64		ndts_periodic_gc_runs;
 	__u64		ndts_forced_gc_runs;
+	__u64		ndts_table_fulls;
 };
 
 enum {