Bug#886811: Can't do system-wide mounts/unmounts in a ssh sessions

[Marius Vollmer <marius.vollmer@redhat.com>]

Package: openssh-server
Version: 1:7.6p1-2

Mounting or unmounting filesystems from within a ssh session used to
affect all processes, but now it only affects ssh sessions (all of them,
not just the one doing the mounts/unmounts).

To reproduce:

 $ ssh root@debian-box
 # dd if=/dev/zero of=/disk.img bs=1M count=20
 # dev=$(losetup --show --find /disk.img)
 # mkfs.ext4 $dev

 # mount $dev /mnt
 # grep /mnt /proc/self/mounts
 /dev/loop0 /mnt ext4 rw,relatime,data=ordered 0 0
 # grep /mnt /proc/1/mounts
 <empty>

It seems that sshd and all its sessions are now in a single dedicated
"mnt" namespace:

 # lsns
         NS TYPE   NPROCS   PID USER             COMMAND
 ...
 4026532316 mnt         1   508 root             /usr/sbin/sshd -D
 ...