dm integrity: flush the journal on suspend

This commit flushes the journal on suspend. It is prerequisite for the
next commit that enables activating dm integrity devices in read-only mode.

Note that we deliberately didn't flush the journal on suspend, so that the
journal replay code would be tested. However, the dm-integrity code is 5
years old now, so that journal replay is well-tested, and we can make this
change now.

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