inotify: use container_of instead of casting

inotify_free_mark casts directly from an fsnotify_mark_entry to an
inotify_inode_mark_entry.  This works, but should use container_of instead
for future proofing.
Signed-off-by: Eric Paris <eparis@redhat.com>

fs/notify/inotify/inotify_user.c

@@ -550,7 +550,9 @@ void inotify_ignored_and_remove_idr(struct fsnotify_mark_entry *entry,
 /* ding dong the mark is dead */
 static void inotify_free_mark(struct fsnotify_mark_entry *entry)
 {
-	struct inotify_inode_mark_entry *ientry = (struct inotify_inode_mark_entry *)entry;
+	struct inotify_inode_mark_entry *ientry;
+
+	ientry = container_of(entry, struct inotify_inode_mark_entry, fsn_entry);
 
 	kmem_cache_free(inotify_inode_mark_cachep, ientry);
 }