nvmet: use u32 type for the local variable nsid

In function nvmet_max_nsid() we calculate the max nsid by iterating
over the XArray and store it in the variable nsid that has type of
unsigned long.

Since the value of this function is stored into the subsys->max_nsid
which is of type u32, change the local variable nsid type and the return
type of the same function to u32.

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