Kernel: Uncommit a shared COW page when discovering it was unshared

When we hit a COW fault and discover than no other process is sharing
the physical page, we simply remap it r/w and save ourselves the
trouble. When this happens, we can also give back (uncommit) one of our
shared committed COW pages, since we won't be needing it.

We had this optimization before, but I mistakenly removed it in
50472fd69f since I had misunderstood
it to be the reason for a panic.

Kernel/VM/MemoryManager.cpp

@@ -1137,4 +1137,11 @@ NonnullRefPtr<PhysicalPage> CommittedPhysicalPageSet::take_one()
     return MM.allocate_committed_user_physical_page(MemoryManager::ShouldZeroFill::Yes);
 }
 
+void CommittedPhysicalPageSet::uncommit_one()
+{
+    VERIFY(m_page_count > 0);
+    --m_page_count;
+    MM.uncommit_user_physical_pages({}, 1);
+}
+
 }