Currently all mounts are restored in a one mount namespace, then
this namespace is cloned to restore other mount namespaces and
we need another copy of this namespace to clean link remap files.

Let's define terms:
A - the root mount namespace
B - the mount namespace where link remap files are destroyed

Currently we restore all mounts in the B namespace and then
clone it into the A namespace.

But it doesn't work in a case, when we have to open file
descriptores to restore mounts (e g to restore bind-mount
pty slaves), because a file descriptor has to be opened from
a specified mount (which is one of restore mount namespaces).

This patch reworks code so, that all mounts is restored in the A mount
namespace and then the B mount namespace is created. In this case
we can open files from the root mount namespace (A).

$ ./zdtm.py run -t zdtm/static/pty-console --iter 5

====================== Run zdtm/static/pty-console in ns =======================
Start test
Test is SUID
./pty-console --pidfile=pty-console.pid --outfile=pty-console.out
Run criu dump
Run criu restore
Run criu dump
=[log]=> dump/zdtm/static/pty-console/36/2/dump.log
------------------------ grep Error ------------------------
(00.106521) Error (criu/files-reg.c:1132): Can't lookup mount=563 for fd=4 path=/ptmx
(00.106585) Error (criu/cr-dump.c:1325): Dump files (pid: 70) failed with -1
(00.129041) Error (criu/cr-dump.c:1674): Dumping FAILED.
------------------------ ERROR OVER ------------------------
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>