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>