net: mpls: rt_nhn_alive and nh_flags should be accessed using READ_ONCE

The number of alive nexthops for a route (rt->rt_nhn_alive) and the
flags for a next hop (nh->nh_flags) are modified by netdev event
handlers. The event handlers run with rtnl_lock held so updates are
always done with the lock held. The packet path accesses the fields
under the rcu lock. Since those fields can change at any moment in
the packet path, both fields should be accessed using READ_ONCE. Updates
to both fields should use WRITE_ONCE.

Update mpls_select_multipath (packet path) and mpls_ifdown and mpls_ifup
(event handlers) accordingly.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>