Kernel: Avoid allocating under spinlock in ProcessGroup::find_or_create

Avoid allocating while holding the g_process_groups_lock spinlock, it's
a pattern that has a negative effect on performance and scalability,
especially given that it is a global lock, reachable by all processes.

Kernel/ProcessGroup.h

@@ -40,6 +40,8 @@ private:
     {
     }
 
+    static RefPtr<ProcessGroup> from_pgid_nolock(ProcessGroupID);
+
     ProcessGroup* m_prev { nullptr };
     ProcessGroup* m_next { nullptr };