inotify: fix segfault on watching hard links

The call to _start() fills in the dirname, basename, and filename
arguments according to the following rules:

  dir watches: dirname filled

  file watches: dirname and basename filled

  hardlink: filename filled

This doesn't map to how the current inotify backend works very nicely,
so we need to adjust things a bit when creating our "sub" objects.

https://bugzilla.gnome.org/show_bug.cgi?id=755721