net/tap: create netdevice during probing

net/tap: create netdevice during probing

This has three main benefits:
 - tun_alloc is now generic again for any queue,
 - mtu no longer needs to be handled in tap_setup_queue(),
 - an actual netdevice is created as soon as the device is probed.

On top of it, code in eth_dev_tap_create() has been reworked to have a
more logical behavior; initialization can now fail if a remote is
requested but cannot be set up.

Fixes: 2bc06869cd94 ("net/tap: add remote netdevice traffic capture")
Signed-off-by: Pascal Mazon <pascal.mazon@6wind.com>