nvme-pci: use the consistent return type of nvme_pci_iod_alloc_size()

The nvme_pci_iod_alloc_size() should return 'size_t' type to be
consistent with the sizeof return value.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>