Umounting a bind-mounted filesystem is usually no different from umounting any other filesystem. However, in your case you used the option
--rbind instead of just
--bind with the
/sys filesystems, which means you did a recursive mount. From
Remount a subtree and all possible submounts somewhere else (so that its contents are available in both places). See above.
/sys are not just single filesystems, but have several other things mounted in them (for example there are several cgroups mounted below
/sys), which the recursive mount replicates under your chroot mountpoint.
root@localhost:~# mount | grep 'chroot/sys'
sysfs on /media/chroot/sys type sysfs (rw,nosuid,nodev,noexec,relatime)
none on /media/chroot/sys/fs/cgroup type tmpfs (rw,relatime,size=4k,mode=755)
cgroup on /media/chroot/sys/fs/cgroup/cpuset type cgroup (rw,relatime,cpuset)
cgroup on /media/chroot/sys/fs/cgroup/cpu type cgroup (rw,relatime,cpu)
cgroup on /media/chroot/sys/fs/cgroup/cpuacct type cgroup (rw,relatime,cpuacct)
cgroup on /media/chroot/sys/fs/cgroup/blkio type cgroup (rw,relatime,blkio)
cgroup on /media/chroot/sys/fs/cgroup/memory type cgroup (rw,relatime,memory)
These additional mounts are not automatically removed along with their ancestor and are what's preventing you from simply umounting
/media/chroot/sys. You need to umount all of those first before you can remove the