Currently, when we use cgroup freezer to seize the tasks we start freezer
and then without waiting the completion of transition procedure we are
seizing tasks read from freezer @tasks file, using fgets.

This is fragile construction because fgets uses internal buffer and tasks
we've read might be exiting same time while we're freezing them,
the kernel won't freeze these exiting tasks because they are dying
anyway and I fear we might read a pid here which is not even in
our cgroup anymore but reused with another out of cgroup task.

Thus lets do the following: use iterations to freeze tasks waiting
for freezer to change its state and then collect/seize all tasks
in one pass.

For example on container I'm playing with it takes just one iteration

 | (00.013690) cg: Set 1 is criu one
 | (00.013705) freezing processes: 1800000 attempst with 100 ms steps
 | (00.013720) freezer.state=THAWED
 | (00.013795) freezer.state=FREEZING
 | (00.113962) freezer.state=FROZEN
 | (00.113990) freezing processes: 1 attempts done
 | (00.114073) SEIZE 240893 (comm systemd): success
 | (00.114110) Warn  (ptrace.c:121): Unable to interrupt task: 240905 (comm kthreadd/1) (Operation not permitted)
 | (00.114136) Warn  (ptrace.c:121): Unable to interrupt task: 240906 (comm khelper) (Operation not permitted)
 | (00.114155) SEIZE 240969 (comm screen): success
 | (00.114166) SEIZE 240970 (comm sendmail): success
 | (00.114179) SEIZE 240971 (comm sendmail): success
 | (00.114189) SEIZE 240972 (comm saslauthd): success
 | (00.114202) SEIZE 240973 (comm crond): success
 | (00.114211) SEIZE 240974 (comm agetty): success
 | (00.114221) SEIZE 240975 (comm agetty): success
 | ...
Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
Acked-by: Andrew Vagin <avagin@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>