mount: Find NS_ROOT for cr-time mount on restore

After commit 2e8970be "mount: create a mount point
for the root mount namespace in the roots yard", top
of the tree of mount_infos points to the fake mount.
So, when we're looking for appropriate place for
binfmt_misc, we can't find "xxx/proc/sys/fs/binfmt_misc".
Fix that by finding real NS_ROOT manually.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>