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>