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