nvme: verify MNAN value if ANA is enabled

The controller is required to have a non-zero MNAN value if it supports
ANA:

   If the controller supports Asymmetric Namespace Access Reporting, then
   this field shall be set to a non-zero value that is less than or equal
   to the NN value.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>