nvme: add fabrics sysfs attributes

- delete_controller: This attribute allows to delete a controller.
  A driver is not obligated to support it (pci doesn't) so it is
  created only if the driver supports it. The new fabrics drivers
  will support it (essentialy a disconnect operation).

  Usage:
  echo > /sys/class/nvme/nvme0/delete_controller

- subsysnqn: This attribute shows the subsystem nqn of the configured
  device. If a driver does not implement the get_subsysnqn method, the
  file will not appear in sysfs.

- transport: This attribute shows the transport name. Added a "name"
  field to struct nvme_ctrl_ops.

  For loop,
  cat /sys/class/nvme/nvme0/transport
  loop

  For RDMA,
  cat /sys/class/nvme/nvme0/transport
  rdma

  For PCIe,
  cat /sys/class/nvme/nvme0/transport
  pcie

- address: This attributes shows the controller address. The fabrics
  drivers that will implement get_address can show the address of the
  connected controller.

  example:
  cat /sys/class/nvme/nvme0/address
  traddr=192.168.2.2,trsvcid=1023

Signed-off-by: Ming Lin <ming.l@ssi.samsung.com>
Reviewed-by: Jay Freyensee <james.p.freyensee@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>