[BUG] fuse-overlayfs can be tricked to omit directories with same inode

### Reproduction

```bash
mkdir lower upper work merged
mkdir -p lower/a/b lower/b
mount -B lower/a/b lower/b
fuse-overlayfs -o lowerdir=lower,upperdir=upper,workdir=work merged
ls lower
>> a b
ls merged
>> a
```

### Expected result

`merged` should contain both directories.

### The problem

When a new node has the same inode, this code checks if it's a directory, frees it if so, and returns the existing node: https://github.com/containers/fuse-overlayfs/blob/84240e9825033cfeae23237104cb51aca60912f9/main.c#L1194-L1206

This is clearly way too dangerous. However, the problem goes away if the `node_dirp (it)` condition is removed.

Reproduce the command sequence in the issue, then inspect main.c around lines 1194-1206, focusing on the same-inode directory handling. Verify that the merged directory retains both a and b, and ensure the corrected behavior is covered by an appropriate regression test.

Written by the indexing model from the issue text.