Create/grow: improve checks on number of devices.

Create/grow: improve checks on number of devices.

Check on upper limit of number of devices was in the wrong place.
Result was could not create array with more than 27 devices without
explicitly setting metadata, even though default metadata allows more.

Fixed, and also perform check when growing an array.

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