xfrm: Fix warning in xfrm6_tunnel_net_exit.

We need to make sure that all states are really deleted
before we check that the state lists are empty. Otherwise
we trigger a warning.

Fixes: baeb0dbb ("xfrm6_tunnel: exit_net cleanup check added")
Reported-and-tested-by:  <syzbot+777bf170a89e7b326405@syzkaller.appspotmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>