datapath: backport: ovs: propagate per dp max headroom to all vports

datapath: backport: ovs: propagate per dp max headroom to all vports

Upstream commit:
    commit 3a927bc7cf9d0fbe8f4a8189dd5f8440228f64e7
    Author: Paolo Abeni <pabeni@redhat.com>

    ovs: propagate per dp max headroom to all vports

    This patch implements bookkeeping support to compute the maximum
    headroom for all the devices in each datapath. When said value
    changes, the underlying devs are notified via the
    ndo_set_rx_headroom method.

    This also increases the internal vports xmit performance.

    Signed-off-by: Paolo Abeni <pabeni@redhat.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Pravin B Shelar <pshelar@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>