please revert kthread from loop.c

[Hugh Dickins <hugh@veritas.com>]

Please revert c7b2eff059fcc2d1b7085ee3d84b79fd657a537b
[PATCH] kthread: update loop.c to use kthread

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): 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.

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