net: sit: unregister_netdevice on newlink's error path

We need to unregister the netdevice if config failed.
.ndo_uninit takes care of most of the heavy lifting.

This was uncovered by recent commit c269a24c ("net: make
free_netdev() more lenient with unregistering devices").
Previously the partially-initialized device would be left
in the system.

Reported-and-tested-by:  <syzbot+2393580080a2da190f04@syzkaller.appspotmail.com>
Fixes: e2f1f072 ("sit: allow to configure 6rd tunnels via netlink")
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20210114012947.2515313-1-kuba@kernel.org


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