mpls: fix clearing of dead nh_flags on link up

recent fixes to use WRITE_ONCE for nh_flags on link up,
accidently ended up leaving the deadflags on a nh. This patch
fixes the WRITE_ONCE to use freshly evaluated nh_flags.

Fixes: 39eb8cd1

 ("net: mpls: rt_nhn_alive and nh_flags should be accessed using READ_ONCE")
Reported-by: Satish Ashok <sashok@cumulusnetworks.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>