mount: change cwd on the root before restoring mntns (v2)

We are going to make pivot_root after restoring mount name-space,
so relative paths will be used for mountpoints.

v2: print correct root in a error message
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>