linux: Don't watch parent directory when target path already exists (#21854)

The Linux watcher was unconditionally watching the parent directory of
every watched path. This is needed in the case of config files that may
not exist when the watch is set up, but not in other cases. Scoping the
parent watch more narrowly cuts down on the amount of error logging from
irrelevant file change notifications being sent to Zed (in my case it
was picking up changes to a random file in `$HOME`).

Release Notes:

- N/A