nvmet: use NVMET_MAX_NAMESPACES to set nn value

For Spec regarding MNAN value:-

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.

Instead of using subsys->max_nsid that gets calculated dynamically,
use NVMET_MAX_NAMESPACES value to report NN. This way we will maintain
the MNAN value spec compliant with NN.

Without this patch, code results in the following error :-

[337976.409142] nvme nvme1: Invalid MNAN value 1024

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>