mtd: introduce mtd_get_unmapped_area interface

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/drivers/mtd/mtdconcat.c b/drivers/mtd/mtdconcat.c
index 76123bd..b3895cf 100644
--- a/drivers/mtd/mtdconcat.c
+++ b/drivers/mtd/mtdconcat.c
@@ -726,8 +726,8 @@
 			return (unsigned long) -EINVAL;
 
 		if (subdev->get_unmapped_area)
-			return subdev->get_unmapped_area(subdev, len, offset,
-							 flags);
+			return mtd_get_unmapped_area(subdev, len, offset,
+						     flags);
 
 		break;
 	}