We create new `host_paths` with SQL, which can't canonicalize and so can't set the `c14n_path_hash` for the new `host_paths`. This second step does canonicalize, and so we can (and need to) set that field here if it isn't already present. This conditional checks whether:
- the `host_path` has the correct `mapping_id` set already - this could be false for both new and existing `host_paths`
- the `host_path` has its `c14n_path_hash` set to match its `path` - this should only be false for new `host_paths`, because all existing ones should have had it set just after they were created

...and updates the `host_path` if either of those fields are not already up-to-date.

We only document the first of the conditions in this line in names and comments here, so I tend to forget about the reason for the second one.