nat: Keep track of how many connections we reject

Keep track of how many connections we reject due the IMSI
filter itself or due not being able to parse the message.
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index e7c8928..77deef4 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -47,6 +47,9 @@
 	[BCFG_CTR_NET_RECONN]    = { "net.reconnects", "Network reconnects       "},
 	[BCFG_CTR_DROPPED_SCCP]  = { "dropped.sccp",   "Dropped SCCP connections."},
 	[BCFG_CTR_DROPPED_CALLS] = { "dropped.calls",  "Dropped active calls.    "},
+	[BCFG_CTR_REJECTED_CR]   = { "rejected.cr",    "Rejected CR due filter   "},
+	[BCFG_CTR_REJECTED_MSG]  = { "rejected.msg",   "Rejected MSG due filter  "},
+	[BCFG_CTR_ILL_PACKET]    = { "rejected.ill",   "Rejected due parse error "},
 };
 
 static const struct rate_ctr_group_desc bsc_cfg_ctrg_desc = {