vfs: mnt_drop_write_file()

new helper (wrapper around mnt_drop_write()) to be used in pair with
mnt_want_write_file().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/ocfs2/move_extents.c b/fs/ocfs2/move_extents.c
index 1d3bf83..b1e3fce 100644
--- a/fs/ocfs2/move_extents.c
+++ b/fs/ocfs2/move_extents.c
@@ -1145,7 +1145,7 @@
 
 	kfree(context);
 
-	mnt_drop_write(filp->f_path.mnt);
+	mnt_drop_write_file(filp);
 
 	return status;
 }