nanobsd: Change the mode in tgt_dir2symlink()

[jlduran]

Summary

Actually change the directory's mode. While it is enforced in the final
image via the METALOG, the directory under $NANO_WORLDDIR should also be
updated.

This is especially important when using $NANO_WORLDDIR as the mountpoint
for a null-mounted poudriere jail, since var/tmp must be world-writable
for poudriere bulk to succeed.

Fixes: 6eccd84bee75 ("nanobsd: Make tgt_dir2symlink() take a mode")

[jlduran]

I had more time to test this change. I might change it to:

chmod "$mode" "$dir" && chmod -h "$mode" "$dir"

because it produces the "right" mtree, so that when it is a symbolic link, it takes the mode of the symbolic link itself and not the target (that is why I'm inclined to also add chmod -h so both, the target directory and the symlink have the desired mode).

[senguptaangshuman17_gmail.com]

This seems to be necessary as chmod on symlink directory updates the source directory not the symlink one.
But do we need to do chmod "$mode" "$dir"?
The goal of tgt_dir2symlink is to set up the mode of the symlink directory and if we just do chmod -h "$mode" "$dir" will change the mode of the symlink.