openvswitch: Fix FLOW_BUFSIZE definition.

The vlan encapsulation fields in the maximum flow defintion were
never updated when the representation changed before upstreaming.
In theory this could cause a kernel panic when a maximum length
flow is used.  In practice this has never happened (to my knowledge)
because skb allocations are padded out to a cache line so you would
need the right combination of flow and packet being sent to userspace.

Signed-off-by: Jesse Gross <jesse@nicira.com>