dm-raid: silence compiler warning on rebuilds_per_group.

commit 3f6bbd3ffd7b733dd705e494663e5761aa2cb9c1 upstream.

This doesn't really need to be initialised, but it doesn't hurt,
silences the compiler, and as it is a counter it makes sense for it to
start at zero.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>