fsnotify: do not set group for a mark before it is on the i_list

fsnotify_add_mark is supposed to add a mark to the g_list and i_list and to
set the group and inode for the mark.  fsnotify_destroy_mark_by_entry uses
the fact that ->group != NULL to know if this group should be destroyed or
if it's already been done.

But fsnotify_add_mark sets the group and inode before it actually adds the
mark to the i_list and g_list.  This can result in a race in inotify, it
requires 3 threads.

sys_inotify_add_watch("file")	sys_inotify_add_watch("file")	sys_inotify_rm_watch([a])
inotify_update_watch()
inotify_new_watch()
inotify_add_to_idr()
   ^--- returns wd = [a]
				inotfiy_update_watch()
				inotify_new_watch()
				inotify_add_to_idr()
				fsnotify_add_mark()
				   ^--- returns wd = [b]
				returns to userspace;
								inotify_idr_find([a])
								   ^--- gives us the pointer from task 1
fsnotify_add_mark()
   ^--- this is going to set the mark->group and mark->inode fields, but will
return -EEXIST because of the race with [b].
								fsnotify_destroy_mark()
								   ^--- since ->group != NULL we call back
									into inotify_freeing_mark() which calls
								inotify_remove_from_idr([a])

since fsnotify_add_mark() failed we call:
inotify_remove_from_idr([a])     <------WHOOPS it's not in the idr, this could
					have been any entry added later!

The fix is to make sure we don't set mark->group until we are sure the mark is
on the inode and fsnotify_add_mark will return success.
Signed-off-by: Eric Paris <eparis@redhat.com>

fs/notify/inode_mark.c

@@ -324,11 +324,11 @@ int fsnotify_add_mark(struct fsnotify_mark_entry *entry,
 	spin_lock(&group->mark_lock);
 	spin_lock(&inode->i_lock);
 
+	lentry = fsnotify_find_mark_entry(group, inode);
+	if (!lentry) {
 		entry->group = group;
 		entry->inode = inode;
 
-	lentry = fsnotify_find_mark_entry(group, inode);
-	if (!lentry) {
 		hlist_add_head(&entry->i_list, &inode->i_fsnotify_mark_entries);
 		list_add(&entry->g_list, &group->mark_entries);