bbc_envctrl: Clean up properly if kthread_run() fails.

In bbc_envctrl_init() we have to unlink the fan and temp instances
from the lists because our caller is going to free up the 'bp' object
if we return an error.

We can't rely upon bbc_envctrl_cleanup() to do this work for us in
this case.

Reported-by: Patrick Finnegan <pat@computer-refuge.org>
Signed-off-by: David S. Miller <davem@davemloft.net>