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

Rename the normalize_mounts() helper to drop_unused_mounts. All the
helpers called in there get rid of mounts that are unused for a variety
of reasons. And whereas the helpers are aptly prefixed with "drop" the
overall helper isn't and instead uses "normalize".

Make it more obvious what the helper actually does by renaming it from
normalize_mounts() to drop_unused_mounts(). Readers of code calling this
helper will immediately see that it will get rid of unused mounts.

Link: systemd/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. */