dm raid: move _get_reshape_sectors() as prerequisite to fixing reshape size issues

rs_set_dev_and_array_sectors() needs this function to
calculate device and array size properly in case leg data
devices have out-of-place reshape space allocated.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>