The nb_dev field is not needed.
Most uses are the test if it is zero or not, and they can be replaced
by tests on the emptiness of the disks list.

Other uses are for iterating through devices in numerical order and
it makes the code clearer (IMO) to unroll the devices into an array first
(which has to be done at some stage anyway) and then walk that array.

This makes ITERATE_RDEV_ORDERED un-necessary.

Also remove the "name" field which is never used.