criu: check: don't leak the child if PTRACE_ATTACH fails

PS: never ever use PTRACE_KILL.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
criu/testing: caps00: don't hang if PR_CAPBSET_DROP fails

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>