blocklayoutdriver: Fix reference leak of pnfs_device_node

The error path for blocklayout's device lookup is missing a reference drop
for the case where a lookup finds the device, but the device is marked with
NFS_DEVICEID_UNAVAILABLE.

Fixes: b3dce6a2

 ("pnfs/blocklayout: handle transient devices")
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>