Add action=spare-same-slot policy.

Add action=spare-same-slot policy.

When "mdadm -I" is given a device with no metadata, mdadm tries to add
it as a 'spare' somewhere based on policy.

This patch changes the behaviour in two ways:

1/ If the device is at a 'path' where a previous device was removed
  from an array or container, then we preferentially add the spare to
  that array or container.

2/ Previously only 'bare' devices were considered for adding as
  spares.  Now if action=spare-same-slot is active, we will add
  non-bare devices, but *only* if the path was previously in use
  for some array, and the device will only be added to that array.

Based on code
  From: Przemyslaw Czarnowski <przemyslaw.hawrylewicz.czarnowski@intel.com>

Signed-off-by: Przemyslaw Czarnowski <przemyslaw.hawrylewicz.czarnowski@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>