nvme: all namespaces in a subsystem must adhere to a common atomic write size

The first namespace configured in a subsystem sets the subsystem's
atomic write size based on its AWUPF or NAWUPF. Subsequent namespaces
must have an atomic write size (per their AWUPF or NAWUPF) less than or
equal to the subsystem's atomic write size, or their probing will be
rejected.

Signed-off-by: Alan Adamson <alan.adamson@oracle.com>
[hch: fold in review comments from John Garry]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>