ovl: fix nlink leak in ovl_rename()

This patch fixes an overlay inode nlink leak in the case where
ovl_rename() renames over a non-dir.

This is not so critical, because overlay inode doesn't rely on
nlink dropping to zero for inode deletion.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>