openvswitch: Use correct type while allocating flex array.

Flex array is used to allocate hash buckets which is type struct
hlist_head, but we use `struct hlist_head *` to calculate
array size.  Since hlist_head is of size pointer it works fine.

Following patch use correct type.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>