net/tap: add MTU management

net/tap: add MTU management

The MTU is assigned to the tap netdevice according to the argument, but
packet transmission and reception just write/read on an fd with the
default limit being the socket buffer size.

As a new rte_eth_dev_data is allocated during tap device init, ensure it
is set again dev->data->mtu.
Once the actual netdevice is created via tun_alloc(), make sure to apply
the desired MTU to the netdevice.

Signed-off-by: Pascal Mazon <pascal.mazon@6wind.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>