fs: ensure that internal tmpfs mount gets mount id zero

fs: ensure that internal tmpfs mount gets mount id zero

and the rootfs get mount id one as it always has. Before we actually
mount the rootfs we create an internal tmpfs mount which has mount id
zero but is never exposed anywhere. Continue that "tradition".

Link: https://patch.msgid.link/20260112-work-immutable-rootfs-v2-1-88dd1c34a204@kernel.org
Fixes: 7f9bfafc5f49 ("fs: use xarray for old mount id")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>