dm cache: prevent entering passthrough mode after unclean shutdown

dm-cache assumes all cache blocks are dirty when it recovers from an
unclean shutdown. Given that the passthrough mode doesn't handle dirty
blocks, we should not load a cache in passthrough mode if it was not
cleanly shut down; or we'll risk data loss while updating an actually
dirty block.

Also bump the target version to 2.4.0 to mark completion of passthrough
mode fixes.

Reproduce steps:

1. Create a writeback cache with zero migration_threshold to produce
dirty blocks.

dmsetup create cmeta --table "0 8192 linear /dev/sdc 0"
dmsetup create cdata --table "0 131072 linear /dev/sdc 8192"
dmsetup create corig --table "0 262144 linear /dev/sdc 262144"
dd if=/dev/zero of=/dev/mapper/cmeta bs=4k count=1 oflag=direct
dmsetup create cache --table "0 262144 cache /dev/mapper/cmeta \
/dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 writeback smq \
2 migration_threshold 0"

2. Write the first cache block dirty

fio --filename=/dev/mapper/cache --name=populate --rw=write --bs=4k \
--direct=1 --size=64k

3. Ensure the number of dirty blocks is 1. This status query triggers
metadata commit without flushing the dirty bitset, setting up the
unclean shutdown state.

dmsetup status cache | awk '{print $14}'

4. Force reboot, leaving the cache uncleanly shutdown.

echo b > /proc/sysrq-trigger

5. Activate the above cache components, and verify the first data block
remains dirty.

dmsetup create cmeta --table "0 8192 linear /dev/sdc 0"
dmsetup create cdata --table "0 131072 linear /dev/sdc 8192"
dmsetup create corig --table "0 262144 linear /dev/sdc 262144"
dd if=/dev/mapper/cdata of=/tmp/cb0.bin bs=64k count=1
dd if=/dev/mapper/corig of=/tmp/ob0.bin bs=64k count=1
md5sum /tmp/cb0.bin /tmp/ob0.bin # expected to be different

6. Try bringing up the cache in passthrough mode. It succeeds, while the
first cache block was loaded dirty due to unclean shutdown, violates
the passthrough mode's constraints.

dmsetup create cache --table "0 262144 cache /dev/mapper/cmeta \
/dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 passthrough smq 0"
dmsetup status cache | awk '{print $14}'

7. (Optional) Demonstrate the integrity issue: invalidating the dirty
block in passthrough mode doesn't write back the dirty data, causing
data loss.

fio --filename=/dev/mapper/cache --name=invalidate --rw=write --bs=4k \
--direct=1 --size=4k # overwrite the first 4k to trigger invalidation
dmsetup remove cache
dd if=/dev/mapper/corig of=/tmp/ob0new.bin bs=64k count=1
cb0sum=$(dd if=/tmp/cb0.bin bs=4k count=15 skip=1 | md5sum | \
awk '{print $1}')
ob0newsum=$(dd if=/tmp/ob0new.bin bs=4k count=15 skip=1 | md5sum | \
awk '{print $1}')
echo "$cb0sum, $ob0newsum" # remaining 60k should differ (data loss)

Signed-off-by: Ming-Hung Tsai <mtsai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>