md: allow number of drives in raid5 to be reduced

When reshaping a raid5 to have fewer devices, we work from the end of
the array to the beginning.
md_do_sync gives addresses to sync_request that go from the beginning
to the end.  So largely ignore them use the internal state variable
"reshape_progress" to keep track of what to do next.

Never allow the size to be reduced below the minimum (4 for raid6,
3 otherwise).

We require that the size of the array has already been reduced before
the array is reshaped to a smaller size.  This is because simply
reducing the size is an easily reversible operation, while the reshape
is immediately destructive and so is not reversible for the blocks at
the ends of the devices.
Thus to reshape an array to have fewer devices, you must first write
an appropriately small size to md/array_size.

When reshape finished, we remove any drives that are no longer
needed and fix up ->degraded.

Signed-off-by: NeilBrown <neilb@suse.de>