nvme: Implement Write Zeroes

Signed-off-by: Keith Busch <keith.busch@intel.com>
[hch: ported over from qemu-nvme.git to mainline]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>