nexthop: allow nexthop_mpath_fill_node() to be called without RTNL

nexthop_mpath_fill_node() will be potentially called
from contexts holding rcu_lock instead of RTNL.

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/all/ZdZDWVdjMaQkXBgW@shredder/


Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>