datapath: don't call pad_packet if not necessary

datapath: don't call pad_packet if not necessary

Upstream commit:
    commit 61ca533c0e94104c35fcb7858a23ec9a05d78143
    Author: Tonghao Zhang <xiangxia.m.yue@gmail.com>
    Date:   Thu Nov 14 23:51:08 2019 +0800

    net: openvswitch: don't call pad_packet if not necessary

    The nla_put_u16/nla_put_u32 makes sure that
    *attrlen is align. The call tree is that:

    nla_put_u16/nla_put_u32
      -> nla_put		attrlen = sizeof(u16) or sizeof(u32)
      -> __nla_put		attrlen
      -> __nla_reserve	attrlen
      -> skb_put(skb, nla_total_size(attrlen))

    nla_total_size returns the total length of attribute
    including padding.

    Cc: Joe Stringer <joe@ovn.org>
    Cc: William Tu <u9012063@gmail.com>
    Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
    Acked-by: Pravin B Shelar <pshelar@ovn.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
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>