Add reshape restart support for external metadata

Add reshape restart support for external metadata

Patch introduces support for reshape process restart for external metadata
using metadata specific data handling methods.
It introduces recover_backup() function that restores array to stable state
It is equivalent to Grow_restart() functionality for native metadata.

Signed-off-by: Maciej Trela <maciej.trela@intel.com>
Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: Krzysztof Wojcik <krzysztof.wojcik@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>