ptrace: don't call PTRACE_INTERRUPT many times

According to the commit message in the kernel git, a new trap flag
JOBCTL_TRAP_INTERRUPT is added, which is set on PTRACE_INTERRUPT and
cleared when any trap happens.

Currently we have a race window beetwen PTRACE_CONT and
PTRACE_INTERRUPT. If a process stops before PTRACE_INTERRUPT,
the flag JOBCTL_TRAP_INTERRUPT will be set once again.

https://bugzilla.openvz.org/show_bug.cgi?id=2569



Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>