dm thin: initialize dm_thin_new_mapping returned by get_next_mapping

commit 16961b042db8cc5cf75d782b4255193ad56e1d4f upstream.

As additional members are added to the dm_thin_new_mapping structure
care should be taken to make sure they get initialized before use.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>