ipv6: fix race between ipv6_del_addr and DAD timer

Consider the following scenario:

ipv6_del_addr(ifp)
  ipv6_ifa_notify(RTM_DELADDR, ifp)
    ip6_del_rt(ifp->rt)

after returning from the ipv6_ifa_notify and enabling BH-s
back, but *before* calling the addrconf_del_timer the 
ifp->timer fires and:

addrconf_dad_timer(ifp)
  addrconf_dad_completed(ifp)
    ipv6_ifa_notify(RTM_NEWADDR, ifp)
      ip6_ins_rt(ifp->rt)

then return back to the ipv6_del_addr and:

in6_ifa_put(ifp)
  inet6_ifa_finish_destroy(ifp)
    dst_release(&ifp->rt->u.dst)

After this we have an ifp->rt inserted into fib6 lists, but 
queued for gc, which in turn can result in oopses in the
fib6_run_gc. Maybe some other nasty things, but we caught 
only the oops in gc so far.

The solution is to disarm the ifp->timer before flushing the
rt from it.

Signed-off-by: Andrey Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>