datapath: fix GFP flags in rtnl_net_notifyid()

datapath: fix GFP flags in rtnl_net_notifyid()

Upstream commit:
    commit d4e4fdf9e4a27c87edb79b1478955075be141f67
    Author: Guillaume Nault <gnault@redhat.com>
    Date:   Wed Oct 23 18:39:04 2019 +0200

    netns: fix GFP flags in rtnl_net_notifyid()

    In rtnl_net_notifyid(), we certainly can't pass a null GFP flag to
    rtnl_notify(). A GFP_KERNEL flag would be fine in most circumstances,
    but there are a few paths calling rtnl_net_notifyid() from atomic
    context or from RCU critical sections. The later also precludes the use
    of gfp_any() as it wouldn't detect the RCU case. Also, the nlmsg_new()
    call is wrong too, as it uses GFP_KERNEL unconditionally.

    Therefore, we need to pass the GFP flags as parameter and propagate it
    through function calls until the proper flags can be determined.

    In most cases, GFP_KERNEL is fine. The exceptions are:
      * openvswitch: ovs_vport_cmd_get() and ovs_vport_cmd_dump()
        indirectly call rtnl_net_notifyid() from RCU critical section,

      * rtnetlink: rtmsg_ifinfo_build_skb() already receives GFP flags as
        parameter.

    Also, in ovs_vport_cmd_build_info(), let's change the GFP flags used
    by nlmsg_new(). The function is allowed to sleep, so better make the
    flags consistent with the ones used in the following
    ovs_vport_cmd_fill_info() call.

    Found by code inspection.

    Fixes: 9a9634545c70 ("netns: notify netns id events")
    Signed-off-by: Guillaume Nault <gnault@redhat.com>
    Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
    Acked-by: Pravin B Shelar <pshelar@ovn.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Backport the datapath.c portion of this fix.

Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>