net: mpls: Update lfib_nlmsg_size to skip deleted nexthops

A recent commit skips nexthops in a route if the device has been
deleted. Update lfib_nlmsg_size accordingly.
Reported-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: NDavid Ahern <dsa@cumulusnetworks.com>
Acked-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Acked-by: NRobert Shearman <rshearma@brocade.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/mpls/af_mpls.c

@@ -1880,6 +1880,8 @@ static inline size_t lfib_nlmsg_size(struct mpls_route *rt)
 		size_t nhsize = 0;
 
 		for_nexthops(rt) {
+			if (!rtnl_dereference(nh->nh_dev))
+				continue;
 			nhsize += nla_total_size(sizeof(struct rtnexthop));
 			/* RTA_VIA */
 			if (nh->nh_via_table != MPLS_NEIGH_TABLE_UNSPEC)