Correct magic number on umount2

The umount2 call was using the magic constant 2 which is
has a defined and proper macro in mount.h as MNT_DETATCH.

Change-Id: I4ca4a6d31cbf5495c545088e3d90a8894a9f912f
diff --git a/sdcard/sdcard.c b/sdcard/sdcard.c
index 041c37a..893c0dc 100644
--- a/sdcard/sdcard.c
+++ b/sdcard/sdcard.c
@@ -1875,7 +1875,7 @@
     struct fuse fuse;
 
     /* cleanup from previous instance, if necessary */
-    umount2(dest_path, 2);
+    umount2(dest_path, MNT_DETACH);
 
     fd = open("/dev/fuse", O_RDWR);
     if (fd < 0){