core/namespace: s/normalize_mounts()/drop_unused_mounts()

Rename the normalize_mounts() helper to drop_unused_mounts. All the
helper called in there drop mounts that are unused for a variety of
reasons. But the helper it self speaks of "normalizing" mounts which
sounds like paths are simplified etc. Make it more obvious what it does
by renaming it and by documenting it.

Link: systemd#22206

src/core/namespace.c

@@ -1578,7 +1578,14 @@ static size_t namespace_calculate_mounts(
                 ns_info->private_ipc; /* /dev/mqueue */
 }
 
-static void normalize_mounts(const char *root_directory, MountEntry *mounts, size_t *n_mounts) {
+/* Walk all mount entries and dropping any unused mounts. This affects all
+ * mounts:
+ * - that are implicitly protected by a path that has been rendered inaccessible
+ * - whose immediate parent requests the same protection mode as the mount itself
+ * - that are outside of the relevant root directory
+ * - which are duplicates
+ */
+static void drop_unused_mounts(const char *root_directory, MountEntry *mounts, size_t *n_mounts) {
         assert(root_directory);
         assert(n_mounts);
         assert(mounts || *n_mounts == 0);
@@ -1684,7 +1691,7 @@ static int apply_mounts(
                 if (!again)
                         break;
 
-                normalize_mounts(root, mounts, n_mounts);
+                drop_unused_mounts(root, mounts, n_mounts);
         }
 
         /* Now that all filesystems have been set up, but before the
@@ -2250,7 +2257,7 @@ int setup_namespace(
                 if (r < 0)
                         goto finish;
 
-                normalize_mounts(root, mounts, &n_mounts);
+                drop_unused_mounts(root, mounts, &n_mounts);
         }
 
         /* All above is just preparation, figuring out what to do. Let's now actually start doing something. */