nvme-pci: clear the prp2 field when not used

If the prp2 field is not filled in nvme_setup_prp_simple(), the prp2
field is garbage data. According to nvme spec, the prp2 is reserved if
the data transfer does not cross a memory page boundary, so clear it to
zero if it is not used.

Signed-off-by: Lei Rao <lei.rao@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>