fib: cleanups

Use rcu_dereference_rtnl() helper

Change hard coded constants in fib_flag_trans()
 7 -> RTN_UNREACHABLE
 8 -> RTN_PROHIBIT

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>