parasite: don't use a process stack to unmap a parasite blob

Process private memory was dumped at this moment, but a memory
tracker isn't reseted, so we can't use a process memory here.

https://github.com/xemul/criu/issues/203

Cc: Eugene Batalov <eabatalov89@gmail.com>
Cc: Fyodor Bocharov <fbocharov@yandex.ru>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Fixes: 101e3ba49d08 ("mem: reset SOFT_DIRTY bits after dumping of all vmas in the process")
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/parasite-syscall.c

@@ -1138,7 +1138,7 @@ int parasite_unmap(struct parasite_ctl *ctl, unsigned long addr)
 	pid_t pid = ctl->pid.real;
 	int ret = -1;
 
-	ret = parasite_run(pid, PTRACE_SYSCALL, addr, NULL, &regs, &ctl->orig);
+	ret = parasite_run(pid, PTRACE_SYSCALL, addr, ctl->rstack, &regs, &ctl->orig);
 	if (ret)
 		goto err;