datapath: remove another BUG_ON()

datapath: remove another BUG_ON()

Upstream commit:
    commit 8a574f86652a4540a2433946ba826ccb87f398cc
    Author: Paolo Abeni <pabeni@redhat.com>
    Date:   Sun Dec 1 18:41:25 2019 +0100

    openvswitch: remove another BUG_ON()

    If we can't build the flow del notification, we can simply delete
    the flow, no need to crash the kernel. Still keep a WARN_ON to
    preserve debuggability.

    Note: the BUG_ON() predates the Fixes tag, but this change
    can be applied only after the mentioned commit.

    v1 -> v2:
     - do not leak an skb on error

    Fixes: aed067783e50 ("openvswitch: Minimize ovs_flow_cmd_del critical section.")
    Signed-off-by: Paolo Abeni <pabeni@redhat.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
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>