Kernel: Guard Process "protected data" with a spinlock

This ensures that both mutable and immutable access to the protected
data of a process is serialized.

Note that there may still be multiple TOCTOU issues around this, as we
have a bunch of convenience accessors that make it easy to introduce
them. We'll need to audit those as well.

Kernel/Syscalls/disown.cpp

@@ -17,8 +17,9 @@ ErrorOr<FlatPtr> Process::sys$disown(ProcessID pid)
         return ESRCH;
     if (process->ppid() != this->pid())
         return ECHILD;
-    ProtectedDataMutationScope scope(*process);
-    process->m_protected_values.ppid = 0;
+    process->with_mutable_protected_data([](auto& protected_data) {
+        protected_data.ppid = 0;
+    });
     process->disowned_by_waiter(*this);
     return 0;
 }