Kernel/NVMeQueue: Use waitqueue in submit_sync_sqe

The current way we handle sync commands is very ugly and depends on lot
of preconditions. Now that we have an end_io handler for a request, we
can use WaitQueue to do sync commands more elegantly.

This does depend on block layer sending one request at a time but this
change is a step forward towards better IO handling.

Kernel/Storage/NVMe/NVMeQueue.h

@@ -95,6 +95,7 @@ private:
     OwnPtr<Memory::Region> m_sq_dma_region;
     Vector<NonnullRefPtr<Memory::PhysicalPage>> m_sq_dma_page;
     Span<NVMeCompletion> m_cqe_array;
+    WaitQueue m_sync_wait_queue;
     Memory::TypedMapping<DoorbellRegister volatile> m_db_regs;
     NonnullRefPtr<Memory::PhysicalPage const> m_rw_dma_page;
 };