nvmet: Change max_nsid in subsystem due to ns_disable if needed

In case we disable namespaces which has the nsid like
subsystem max_nsid we need to search for the next largest nsid
in this subsystem. If the subsystem don't has more namespaces
we set it to 0, else we take nsid from the last namespace in
namespaces list because the list is sorted while inserting.

Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Roy Shterman <roys@lightbitslabs.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
[hch: slight refactor]
Signed-off-by: Christoph Hellwig <hch@lst.de>