datapath: do not update max_headroom if new headroom is equal to old headroom

datapath: do not update max_headroom if new headroom is equal to old headroom

Upstream commit:
    commit 6b660c4177aaebdc73df7a3378f0e8b110aa4b51
    Author: Taehee Yoo <ap420073@gmail.com>
    Date:   Sat Jul 6 01:08:09 2019 +0900

    net: openvswitch: do not update max_headroom if new headroom is equal to old headroom

    When a vport is deleted, the maximum headroom size would be changed.
    If the vport which has the largest headroom is deleted,
    the new max_headroom would be set.
    But, if the new headroom size is equal to the old headroom size,
    updating routine is unnecessary.

    Signed-off-by: Taehee Yoo <ap420073@gmail.com>
    Tested-by: Greg Rose <gvrose8192@gmail.com>
    Reviewed-by: Greg Rose <gvrose8192@gmail.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>