devmapper: Fix comments and for HasDevice() and Exists()

Exists() and HasDevice() just check if device file exists or not. It does
not say anything about if device is mounted or not. Fix comments.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

daemon/graphdriver/devmapper/deviceset.go

@@ -1786,7 +1786,7 @@ func (devices *DeviceSet) UnmountDevice(hash string) error {
 	return nil
 }
 
-// HasDevice returns true if the device is in the hash and mounted.
+// HasDevice returns true if the device metadata exists.
 func (devices *DeviceSet) HasDevice(hash string) bool {
 	devices.Lock()
 	defer devices.Unlock()

daemon/graphdriver/devmapper/driver.go

@@ -196,7 +196,7 @@ func (d *Driver) Put(id string) error {
 	return err
 }
 
-// Exists checks to see if the device is mounted.
+// Exists checks to see if the device exists.
 func (d *Driver) Exists(id string) bool {
 	return d.DeviceSet.HasDevice(id)
 }