imsm: update metadata correctly while raid10 double degradation

imsm: update metadata correctly while raid10 double degradation

Mdmon calls end_migration() when map state changes from normal to
degraded. It is not valid because in raid 10 double degradation case
mdmon breaks checkpointing but array is still rebuilding.
In this case mdmon has to mark map as degraded and continues marking
recovery checkpoint in metadata. Migration can be finished only if newly
failed device is a rebuilding device.

Add catching double degraded to degraded transition. Migration is
finished but map state doesn't change, array is still degraded.

Update failed_disk_num correctly. If double degradation
happens rebuild will start on the lowest slot, but this variable points
to the first failed slot. If second fail happens while rebuild this
variable shouldn't be updated until rebuild is not finished.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>