thin: fix link validation for unused thin-pool

thin: fix link validation for unused thin-pool

This patch fixes link validation for used thin-pool.
Udev rules correctly creates symlinks only for unused new thin-pool.
Such thin-pool can be used by foreing apps (like Docker) thus
has  /dev/vg/lv link.
However when thin-pool becomes used by thinLV - this link is no
longer exposed to user - but internal verfication missed this
and caused messages like this to be printed upon 'vgchange -ay':

The link /dev/vg/pool should have been created by udev but it was not
found. Falling back to direct link creation.

And same with 'vgchange -an':
The link /dev/vg/pool should have been removed by udev but it is still
present. Falling back to direct link removal.

This patch ensures only unused thin-pool has this link.