[SPARC]: Use kthread infrastructure in envctrl

envctrl currently uses very odd ways to stop a thread, using various
things that should be exposed to drivers at all.

This patch (which is untested as I don't have sparc hardware) switches
it to use the proper kthread infrastructure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>