ovl: fix return value of ovl_fill_super

If kcalloc() failed, the return value of ovl_fill_super() is -EINVAL,
not -ENOMEM. So this patch sets this value to -ENOMEM before calling
kcalloc(), and sets it back to -EINVAL after calling kcalloc().

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>