Kernel/Tasks: Allow Kernel processes to be shut down

Since we never check a kernel process's state like a userland process,
it's possible for a kernel process to ignore the fact that someone is
trying to kill it, and continue running. This is not desireable if we
want to properly shutdown all processes, including Kernel ones.

Kernel/Tasks/SyncTask.cpp

@@ -16,10 +16,12 @@ UNMAP_AFTER_INIT void SyncTask::spawn()
 {
     MUST(Process::create_kernel_process(KString::must_create("VFS Sync Task"sv), [] {
         dbgln("VFS SyncTask is running");
-        for (;;) {
+        while (!Process::current().is_dying()) {
             VirtualFileSystem::sync();
             (void)Thread::current()->sleep(Duration::from_seconds(1));
         }
+        Process::current().sys$exit(0);
+        VERIFY_NOT_REACHED();
     }));
 }