mount: execute propagation logic for the root mount (v4)

The root mount isn't always private. For example it is mounted
as a slave in LXC 1.0 containers. So we need to execute logic
about propogation for the root mount too.

v2: move all logic about the root mount in a separate function
v3: make code more readable
v4: do_mount_root() looks like other do_*_root() functions

Reported-by: David Shwatrz <dshwatrz@gmail.com>
Cc: David Shwatrz <dshwatrz@gmail.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>