pidfs: add some CONFIG_DEBUG_VFS asserts

Allow to catch some obvious bugs.

Link: https://lore.kernel.org/20250618-work-pidfs-persistent-v2-16-98f3456fd552@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/pidfs.c

@@ -809,6 +809,8 @@ static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
 	if (ret < 0)
 		return ERR_PTR(ret);
 
+	VFS_WARN_ON_ONCE(!pid->attr);
+
 	mntput(path.mnt);
 	return path.dentry;
 }
@@ -1038,6 +1040,8 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
 	if (ret < 0)
 		return ERR_PTR(ret);
 
+	VFS_WARN_ON_ONCE(!pid->attr);
+
 	flags &= ~PIDFD_STALE;
 	flags |= O_RDWR;
 	pidfd_file = dentry_open(&path, flags, current_cred());