ethdev: MTU accessors

ethdev: MTU accessors

This patch adds two new functions in ethdev api to retrieve current MTU and
change MTU of a port.
Only .mtu_set function is pmd specific.
pmd should update its max_rx_pkt_len if needed.

This operation has been implemented for rte_em_pmd, rte_igb_pmd and
rte_ixgbe_pmd.

Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>
Signed-off-by: Ivan Boule <ivan.boule@6wind.com>
Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>