monitor_testcase8: Checkpointing

Check if the final state of the MD array is identical to the
initial one.

Signed-off-by: Hannes Reinecke <hare@suse.de>
md_monitor: Fixup locking sequence in reset_mirror()

reset_mirror() would use a different locking sequence than
fail_mirror(), causing fail_mirror() to be invoked errorneously.

References: bnc#840526

Signed-off-by: Hannes Reinecke <hare@suse.de>