configfs: calculate the depth of parent item

When create symbolic link, create_link should calculate the depth
of the parent item. However, both the first and second parameters
of configfs_get_target_path had been set to the target. Broken
symbolic link created.

$ targetcli ls /
o- / ............................................................. [...]
  o- backstores .................................................. [...]
  | o- block ...................................... [Storage Objects: 0]
  | o- fileio ..................................... [Storage Objects: 2]
  | | o- vdev0 .......... [/dev/ramdisk1 (16.0MiB) write-thru activated]
  | | | o- alua ....................................... [ALUA Groups: 1]
  | | |   o- default_tg_pt_gp ........... [ALUA state: Active/optimized]
  | | o- vdev1 .......... [/dev/ramdisk2 (16.0MiB) write-thru activated]
  | |   o- alua ....................................... [ALUA Groups: 1]
  | |     o- default_tg_pt_gp ........... [ALUA state: Active/optimized]
  | o- ps...