We have a problem when a pid is reused between consequent dumps we can't
understand if pagemap and pages from images of parent dump are invalid
to restore these pid already. That can lead even to wrong memory
restored for these pid, see the test in last patch.

So these is a try do separate processes with (likely) invalid previous
memory dump from processes with 100% valid previous dump.

For that we use the value of /proc/<pid>/stat's start_time and also the
timestamp of each (pre)dump. If the start time is strictly less than the
timestamp, that means that the pagemap for these pid from previous dump
is valid - was done for exactly the same process.

Creation time is in centiseconds by default so if predump is really fast
(<1csec) we can have false negative decisions for some processes, but in
case of long running processes we are fine.

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

v2: remove __maybe_unused for get_parent_stats; fix get_parent_stats to
have static typing; print warning only if unsure; check has_dump_uptime
v3: read parent stats from image only once; reuse stat from previous
parse_pid_stat call on dump
v4: move code to function; use unsigned long long for ticks; put
proc_pid_stat on mem_dump_ctl; print warning on all pid-reuse cases
v5: free parent's stats entry properly, pass it in arguments to
(pre_)dump_one_task
v6: free parent's stats in error path too
v7: zero init parent_se
v8: improve error message
v9: switch to inventory image from stats, if pid-reuse fails - fail
current dump
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>