Fix go vet errors

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)

daemon/graphdriver/devmapper/deviceset.go

@@ -55,7 +55,7 @@ type DevInfo struct {
 }
 
 type MetaData struct {
-	Devices     map[string]*DevInfo `json:devices`
+	Devices     map[string]*DevInfo `json:"Devices"`
 	devicesLock sync.Mutex          `json:"-"` // Protects all read/writes to Devices map
 }