ethdev: make fast-path functions to use new flat array

ethdev: make fast-path functions to use new flat array

Rework fast-path ethdev functions to use rte_eth_fp_ops[].
While it is an API/ABI breakage, this change is intended to be
transparent for both users (no changes in user app is required) and
PMD developers (no changes in PMD is required).
One extra thing to note - RX/TX callback invocation will cause extra
function call with these changes. That might cause some insignificant
slowdown for code-path where RX/TX callbacks are heavily involved.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Tested-by: Feifei Wang <feifei.wang2@arm.com>