openvswitch: Use kmem_cache_free() instead of kfree()

memory allocated by kmem_cache_alloc() should be freed using
kmem_cache_free(), not kfree().

Fixes: e298e505 ('openvswitch: Per cpu flow stats.')
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>