proc: block mounting on top of /proc/<pid>/fdinfo/*

Entries under /proc/<pid>/fdinfo/* are ephemeral and may go away before
the process dies. As such allowing them to be used as mount points
creates the ability to leak mounts that linger until the process dies
with no ability to unmount them until then. Don't allow using them as
mountpoints.

Link: https://lore.kernel.org/r/20240806-work-procfs-v1-6-fb04e1d09f0c@kernel.org
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/proc/fd.c

@@ -397,8 +397,8 @@ static struct dentry *proc_fdinfo_instantiate(struct dentry *dentry,
 	inode->i_fop = &proc_fdinfo_file_operations;
 	tid_fd_update_inode(task, inode, 0);
 
-	d_set_d_op(dentry, &tid_fd_dentry_operations);
-	return d_splice_alias(inode, dentry);
+	return proc_splice_unmountable(inode, dentry,
+				       &tid_fd_dentry_operations);
 }
 
 static struct dentry *