Kernel: Fix assertion when releasing contiguous memory region

There is no guarantee that the memory manager lock is held when
physical pages are released, so just acquire the memory manager
lock.

Kernel/VM/MemoryManager.cpp

@@ -453,7 +453,7 @@ RefPtr<PhysicalPage> MemoryManager::allocate_user_physical_page(ShouldZeroFill s
 
 void MemoryManager::deallocate_supervisor_physical_page(PhysicalPage&& page)
 {
-    ASSERT(s_mm_lock.is_locked());
+    ScopedSpinLock lock(s_mm_lock);
     for (auto& region : m_super_physical_regions) {
         if (!region.contains(page)) {
             klog() << "MM: deallocate_supervisor_physical_page: " << page.paddr() << " not in " << region.lower() << " -> " << region.upper();