Revert "[PATCH] kthread: update loop.c to use kthread"

This reverts commit c7b2eff059.

Hugh Dickins explains:

 "It seems too little tested: "losetup -d /dev/loop0" fails with
  EINVAL because nothing sets lo_thread; but even when you patch
  loop_thread() to set lo->lo_thread = current, it can't survive
  more than a few dozen iterations of the loop below (with a tmpfs
  mounted on /tst):

	j=0
	cp /dev/zero /tst
	while :
	do
	    let j=j+1
	    echo "Doing pass $j"
	    losetup /dev/loop0 /tst/zero
	    mkfs -t ext2 -b 1024 /dev/loop0 >/dev/null 2>&1
	    mount -t ext2 /dev/loop0 /mnt
	    umount /mnt
	    losetup -d /dev/loop0
	done

  it collapses with failed ioctl then BUG_ON(!bio).

  I think the original lo_done completion was more subtle and safe
  than the kthread conversion has allowed for."

Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/loop.h

@@ -59,7 +59,7 @@ struct loop_device {
 	struct bio 		*lo_bio;
 	struct bio		*lo_biotail;
 	int			lo_state;
-	struct task_struct	*lo_thread;
+	struct completion	lo_done;
 	struct completion	lo_bh_done;
 	struct mutex		lo_ctl_mutex;
 	int			lo_pending;