We need to open a file to restore a file mapping and this file
can be from a current mntns.

v2: All namespaces are resotred from the root task and then
other tasks calls setns() to set a proper mntns.

v3: fix comments from Pavel
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>