Support restore_stripes() from the given buffer

Support restore_stripes() from the given buffer

For external metadata backup location and saving methods depends
on metadata specific implementation details. Currently restore_stripes()
function is able to restore data only from the given backup file handles
and it is used only for assembling partially reshaped arrays.
As this function will be very helpful for external metadata backup
mechanism, add the support for restoring data from the given source buffer.
Add possibility for save_stripes() to work without designation targets.
Save_stripes() can now prepare data for restore_stripes() only.

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>