nvme-pci: use struct_size for allocation struct nvme_dev

This avoids open coding the variable size array arithmetics.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Leon Romanovsky <leon@kernel.org>