Fix error discarding device when delete volumes
Fix error discarding device when delete volumes

When deleting volumes, convoy daemon will always log error
messages like following:

"Error open volume-e4956a83f67147c0: no such file or directory
 when discarding volume-e4956a83f67147c0, ignored  pkg=devmapper"

This patch fixes it by passing correct argument to the API
devicemapper.BlockDeviceDiscard which expects a full path of
a device instead of just device name.

Signed-off-by: Jin Xu <jinuxstyle@hotmail.com>
Reviewed-by: Sheng Yang <sheng.yang@rancher.com>

V2: use filepath.Join to generate the full path

rancherio/rancher-volume,medallia/convoy,rancher/convoy,rancherio/volmgr,rancherio/volmgr,nickvanw/convoy,rancher/convoy,rancherio/volmgr,rancherio/rancher-volume,rancher/convoy,medallia/convoy,rancher/rancher-volume,rancher/rancher-volume,rancherio/rancher-volume,nickvanw/convoy,medallia/convoy,nickvanw/convoy,rancher/rancher-volume