mbuf: extend mbuf pool private structure

With the API and ABI freeze ahead, it will be good to reserve
some bits on the private structure for future use.

Otherwise we will potentially need to maintain two different
private structure during 2020 period.

There is already one use case for those reserved bits[1]

The reserved field should be set to 0 by the user.

[1] https://patches.dpdk.org/patch/63077/

Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>