nvme-pci: allocate nvme_command within driver pdu

Except for pci, all the nvme transport drivers allocate a command within
the driver's pdu. Align pci with everyone else by allocating the nvme
command within pci's pdu and replace the .queue_rq() stack variable with
this.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>