xfrm: Increment statistic counter on inner mode error

Increment the LINUX_MIB_XFRMINSTATEMODEERROR statistic counter
to notify about dropped packets if we fail to fetch a inner mode.
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_input.c

@@ -330,8 +330,10 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
 
 		if (x->sel.family == AF_UNSPEC) {
 			inner_mode = xfrm_ip2inner_mode(x, XFRM_MODE_SKB_CB(skb)->protocol);
-			if (inner_mode == NULL)
+			if (inner_mode == NULL) {
+				XFRM_INC_STATS(net, LINUX_MIB_XFRMINSTATEMODEERROR);
 				goto drop;
+			}
 		}
 
 		if (inner_mode->input(x, skb)) {