openvswitch: Use proper buffer size in nla_memcpy

For the input parameter count, it's better to use the size
of destination buffer size, as nla_memcpy would take into
account the length of the source netlink attribute when
a data is copied from an attribute.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>