ovl: do not set overlay.opaque on non-dir create

The optimization for opaque dir create was wrongly being applied
also to non-dir create.

Fixes: 97c684cc ("ovl: create directories inside merged parent opaque")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org> # v4.10