restorer: unblock only SIGCHLD previously blocked in criu

The purpose is to unblock previously blocked SIGCHLD, not all the
signals. IIRC, this block-unblock dancing is about a race where SIGCHLD
may come during switching to restorer blob, where CRIU handler function
is already unmapped - which will result in segmentation violation if
SIGCHLD wasn't blocked.

Fixes:3a61c38a ("restore: block sigchld to remap task_entries")

Cc: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/pie/restorer.c

@@ -1102,7 +1102,7 @@ long __export_restore_task(struct task_restore_args *args)
 	act.rt_sa_restorer = cr_restore_rt;
 	sys_sigaction(SIGCHLD, &act, NULL, sizeof(k_rtsigset_t));
 
-	ksigfillset(&to_block);
+	ksigemptyset(&to_block);
 	ksigaddset(&to_block, SIGCHLD);
 	ret = sys_sigprocmask(SIG_UNBLOCK, &to_block, NULL, sizeof(k_rtsigset_t));