pidfs: support FS_IOC_GETVERSION

This will allow 32 bit userspace to detect when a given inode number has
been recycled and also to construct a unique 64 bit identifier.

Link: https://lore.kernel.org/r/20241129-work-pidfs-v2-3-61043d66fbce@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>