ethdev: reserve space in main structs for extension

ethdev: reserve space in main structs for extension

In order to allow smooth addition of features without breaking
ABI compatibility, some space is reserved in several core structs
of ethdev API.

The struct rte_eth_dev and rte_eth_dev_data are supposed
to be used internally only, but there is a chance that
increasing their size would break ABI for some applications.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>