Unfortunatelly, there may be bios coming even after the reboot notifier
was called.  We don't want these bios to make the bitmap dirty again.

To address this, implement a synchronous mode - when a bio is about to
be terminated, we clean the bitmap and terminate the bio after the clean
operation succeeds.  This obviously slows down bio processing, but it
makes sure that when all bios are finished, the bitmap will be clean.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>