dm integrity: flush journal on suspend when using separate metadata device

Flush the journal on suspend when using separate data and metadata devices,
so that the metadata device can be discarded and the table can be reloaded
with a linear target pointing to the data device.

NOTE: the journal is deliberately not flushed when using the same device
for metadata and data, so that the journal replay code is tested.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>