datapath: Fix FLOW_BUFSIZE definition.

datapath: Fix FLOW_BUFSIZE definition.

This is analogous to the change made in userspace with
2508ac16defd417b94fb69689b6b1da4fbc76282 (odp-util: Update
ODPUTIL_FLOW_KEY_BYTES for current kernel flow format.).  The extra
space for vlan encapsulation was not included in the allocation for
maximum length flows.

Found by code inspection and to my knowledge has never been hit, likely
because skb allocations are padded out to a cacheline, making userspace
more susceptible to this problem than the kernel.  In theory, however,
the right combination of flow and packet size could result in a kernel
panic.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>