parasite: add a comment why we can safly trap task on sigreturn

The kernel notifies about leaving syscall before starting to deliver
signals. If you don't believe me, pls look at arch/x86/kernel/entry_64.S:

int_ret_from_sys_call
        syscall_trace_leave
        do_notify_resume
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

parasite-syscall.c

@@ -754,6 +754,12 @@ static int parasite_fini_seized(struct parasite_ctl *ctl)
 	if (parasite_stop_on_syscall(1, __NR_rt_sigreturn))
 		return -1;
 
+	/*
+	 * All signals are unblocked now. The kernel notifies about leaving
+	 * syscall before starting to deliver signals. All parasite code are
+	 * executed with blocked signals, so we can sefly unmap a parasite blob.
+	 */
+
 	return 0;
 }