FsMagic should avoid sign extension on i686

`uint64(buf.Type)` on i686 is ffffffff9123683e on i686 due to sign extension, so it cannot be compared with `FsMagic(0x9123683E)`

Signed-off-by: Andrii Melnykov <andy.melnikov@gmail.com>

daemon/graphdriver/driver.go

@@ -11,7 +11,7 @@ import (
 	"github.com/docker/docker/pkg/archive"
 )
 
-type FsMagic uint64
+type FsMagic uint32
 
 const (
 	FsMagicBtrfs = FsMagic(0x9123683E)