nvmet: use u32 for nvmet_subsys max_nsid

Use u32 type for the nsid_max member of the nvmet_subsys structure.
This avoids the type confusion when updating the subsys->nax_nsid from
ns->nsid. This also matches the nvmet_ns->nsid member.

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