improve logic for getting the watcher for a shared external storage

[icewind1991]

with the new-ish lazy mount loading the underlying mount might not be loaded yet, so ask the mount cache instead

To test

• Setup an instance with user A and user B
• mount external storage M for user A
• have user A share M with B
• modify the data in M directly (bypassing NC)
• as B check if the file listing is updated

[come-nc]

(did not test)