Make btrfs call same interface as others

Most storage drivers call graphdriver.GetFSMagic(home),
it is more clean to easy to maintain. So btrfs need to
adopt such change.

Signed-off-by: Kai Qiang Wu(Kennan) <wkqwu@cn.ibm.com>

daemon/graphdriver/btrfs/btrfs.go

@@ -31,14 +31,13 @@ func init() {
 // Init returns a new BTRFS driver.
 // An error is returned if BTRFS is not supported.
 func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (graphdriver.Driver, error) {
-	rootdir := path.Dir(home)
 
-	var buf syscall.Statfs_t
-	if err := syscall.Statfs(rootdir, &buf); err != nil {
+	fsMagic, err := graphdriver.GetFSMagic(home)
+	if err != nil {
 		return nil, err
 	}
 
-	if graphdriver.FsMagic(buf.Type) != graphdriver.FsMagicBtrfs {
+	if fsMagic != graphdriver.FsMagicBtrfs {
 		return nil, graphdriver.ErrPrerequisites
 	}