proc: parse /proc/pid/status when a task is stopped (v2)

If we parse /proc/pid/status when a task isn't stopped,
we can't be sure that a process state will not be changed.

08:58:48 Test: zdtm/live/user/static/zombie00, Namespace: 1
08:58:48 Dump log   : /var/lib/jenkins/jobs/CRIU-dump/workspace/test/dump/ns/user/static/zombie00/114/1/dump.log
08:58:48 --------------------------------- grep Error ---------------------------------
08:58:48 (00.001127) Error (ptrace.c:124): SEIZE 121: task not stopped after seize

v2: don't believe into errno (by xemul@)

Reported-by: Mr Jenkins
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>