md/raid5: enhance raid5_size to work correctly with negative delta_disks

This is the first of four patches which combine to allow md/raid5 to
reduce the number of devices in the array by restriping the data over
a subset of the devices.

If the number of disks in a raid4/5/6 is being reduced, then the
default size must be based on the new number, not the old number
of devices.
In general, it should be based on the smaller of new and old.

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