openvswitch: change type of UPCALL_PID attribute to NLA_UNSPEC

userspace openvswitch patch "(dpif-linux: Implement the API
functions to allow multiple handler threads read upcall)"
changes its type from U32 to UNSPEC, but leave the kernel
unchanged

and after kernel 6e237d09 "(netlink: Relax attr validation
for fixed length types)", this bug is exposed by the below
warning

	[   57.215841] netlink: 'ovs-vswitchd': attribute type 5 has an invalid length.

Fixes: 5cd667b0

 ("openvswitch: Allow each vport to have an array of 'port_id's")
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>