bcache does not work with suspend

Status changed to 'Confirmed' because the bug affects multiple users.

## This uses qemu and a maas image to easily test bcache of a root disk
## on a vm. Originally put together for bug 1515780
## It does:
## * downloads a maas image, kernel and initramfs
## * and makes a disk with 1M in front for the bcache header (root.img)
## * creates a disk for cache image (cache0.img)
## * puts a bcache header on the root.img pointing to cache disk by cset uuid
## * boots system with uuid of root filesystem
##
## once booted you can login with 'ubuntu:passw0rd' (from the user-data above)
## suspend with:
## sudo systemctl suspend
## unsuspend with:
## 'system_wakeup'
## to issue system_wakeup you need to get to qemu monitor.
## in nographic mode you you toggle between console and qemu monitor with
## the provided 'escape char' (-echr) and 'c'

$ wget http://images.maas.io/ephemeral-v2/daily/xenial/amd64/20160509/root-image.gz
$ wget http://images.maas.io/ephemeral-v2/daily/xenial/amd64/20160509/xenial/generic/boot-initrd
$ wget http://images.maas.io/ephemeral-v2/daily/xenial/amd64/20160509/xenial/generic/boot-kernel

$ sudo apt-get install bcache-tools

## set up the cache disk
$ truncate --size 500M cache0.img
$ make-bcache --cache --writeback cache0.img
UUID: 48320c10-9065-40e1-93cc-d020861e2070
Set UUID: 1d997f00-4d73-40d6-9a09-c9192bf6aed2
version: 0
nbuckets: 1000
block_size: 8
bucket_size: 1024
nr_in_set: 1
nr_this_dev: 0
first_bucket: 1
$ cset_uuid=$(bcache-super-show cache0.img | awk '$1 == "cset.uuid" { print $2 }')
$ echo $cset_uuid
1d997f00-4d73-40d6-9a09-c9192bf6aed2

## setup primary disk
$ truncate --size=1M root.img
$ zcat root-image.gz >> root.img

# 2048 is 1M / 512B sectors
$ make-bcache --bdev --data-offset=2048 "--cset-uuid=${cset_uuid}" root.img

# 1M here is same as our truncate. get the UUID of the filesystem.
$ uuid=$(blkid -p -o export -O 1M root.img | awk -F= '$1 == "UUID" { print $2 }')

## set up a seed disk
$ cat > user-data <<EOF
#cloud-config
password: passw0rd
chpasswd: { expire: False}
EOF

$ cloud-localds -v seed.img user-data

## boot it
$ qemu-system-x86_64 -enable-kvm \
   -device virtio-net-pci,netdev=net00 -netdev type=user,id=net00 \
   -drive if=virtio,file=root.img,format=raw \
   -drive if=virtio,file=cache0.img,format=raw \
   -drive if=virtio,file=seed.img,format=raw \
   -nographic \
   -kernel boot-kernel -initrd boot-initrd \
   -append "root=UUID=${uuid} console=ttyS0" -m 1024 -echr 0x05 -snapshot

attaching the version that i've been using successfully for the past 6 months (timestamp on the file is Sep 16, 2016) on a laptop with home on a bcache device.