hw/nvme: fix narrowing conversion

hw/nvme: fix narrowing conversion

Since nlbas is of type int, it does not work with large namespace size
values, e.g., 9 TB size of file backing namespace and 8 byte metadata
with 4096 bytes lbasz gives negative nlbas value, which is later
promoted to negative int64_t type value and results in negative
ns->moff which breaks namespace

Signed-off-by: Dmitry Tikhov <ddtikhov@gmail.com>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>