net/tun: Call type change netdev notifiers

Call netdev notifiers before and after changing the device type.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Link: https://lore.kernel.org/r/20201118063919.29485-1-ms@dev.tdt.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>