stat-util: rename files_same() → inode_same()

Let's be more accurate about what this function does: it checks whether
the underlying reported inode is the same. Internally, this already uses
a better named stat_inode_same() call, hence let's similarly name the
wrapping function following the same logic.

Similar for files_same_at() and path_equal_or_same_files().

No code changes, just some renaming.

src/shared/switch-root.c

@@ -45,7 +45,7 @@ int switch_root(const char *new_root,
         if (new_root_fd < 0)
                 return log_error_errno(errno, "Failed to open target directory '%s': %m", new_root);
 
-        r = files_same_at(old_root_fd, "", new_root_fd, "", AT_EMPTY_PATH);
+        r = inode_same_at(old_root_fd, "", new_root_fd, "", AT_EMPTY_PATH);
         if (r < 0)
                 return log_error_errno(r, "Failed to determine if old and new root directory are the same: %m");
         if (r > 0) {