ovl: fix dentry leak for default_permissions

When using the 'default_permissions' mount option, ovl_permission() on
non-directories was missing a dput(alias), resulting in "BUG Dentry still
in use".

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 8d3095f4 ("ovl: default permissions")
Cc: <stable@vger.kernel.org> # v4.5+