Fix #2020: use /sys to map nvmeXnY to nvmeX

Fix #2020: use /sys to map nvmeXnY to nvmeX

`nvmeX` devices nodes are apparently allocated independently
from their namespace block devices `nvmeXnY` and therefore
they are not strictly related by name. For instance:
  $ readlink /sys/block/nvme0n1/device
  ../../nvme1
  $ readlink /sys/block/nvme1n1/device
  ../../nvme0

Here /dev/nvme0n1 is the first namespace of /dev/nvme1 while
/dev/nvme1n1 is the first namespace of /dev/nvme0.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>