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: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Acked-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>