The idea of the test is:

1) mmap separate page and put variable there, so that other usage does
not dirty these region. Initialize the variable with VALUE_A.

2) fork a child with special pid == CHILD_NS_PID. Only if it is a first
child overwrite the variable with VALUE_B.

3) wait for the end of the next predump or end of restore with
test_wait_pre_dump_ack/test_wait_pre_dump pair and kill our child.

Note: The memory region is "clean" in parent.

4) goto (2) unles end of cr is reported by test_waitpre

So on first iteration child with pid CHILD_NS_PID was dumped with
VALUE_B, on all other iterations and on final dump other child with the
same pid exists but with VALUE_A. But on all iterations after the first
one we have these memory region "clean". So criu before the fix would
have restored the VALUE_B taking it from first child's image, but should
restore VALUE_A.

Note: Child in its turn waits termination and performs a check that variable
value doesn't change after c/r.

We should run the test with at least one predump to trigger the problem:

[root@snorch criu]# ./test/zdtm.py run --pre 1 -k always -t zdtm/transition/pid_reuse
Checking feature ns_pid
Checking feature ns_get_userns
Checking feature ns_get_parent

=== Run 1/1 ================ zdtm/transition/pid_reuse

===================== Run zdtm/transition/pid_reuse in ns ======================
DEP       pid_reuse.d
CC        pid_reuse.o
LINK      pid_reuse
Start test
Test is SUID
./pid_reuse --pidfile=pid_reuse.pid --outfile=pid_reuse.out
Run criu pre-dump
Send the 10 signal to  52
Run criu dump
Run criu restore
Send the 15 signal to  73
Wait for zdtm/transition/pid_reuse(73) to die for 0.100000
Test output: ================================
14:47:57.717: 11235: ERR: pid_reuse.c:76: Wrong value in a variable after restore
14:47:57.717:     4: FAIL: pid_reuse.c:110: Task 11235 exited with wrong code 1 (errno = 11 (Resource temporarily unavailable))

<<< ================================

https://jira.sw.ru/browse/PSBM-67502

v3: simplify waitpid's status check
v9: switch to test_wait_pre_dump(_ack)
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>