Ext2FS: Uncache unused Inodes after flushing contents to disk

Don't keep Inodes around in memory forever after we've interacted with
them once. This is a slight performance pessimization when accessing
the same file repeatedly, but closing it for a while in between.

Longer term we should find a way to keep a limited number of unused
Inodes cached, whichever ones we think are likely to be used again.

Kernel/FileSystem/Ext2FileSystem.cpp

@@ -503,7 +503,21 @@ void Ext2FS::flush_writes()
 #endif
         }
     }
+
     DiskBackedFS::flush_writes();
+
+    // Uncache Inodes that are only kept alive by the index-to-inode lookup cache.
+    // FIXME: It would be better to keep a capped number of Inodes around.
+    //        The problem is that they are quite heavy objects, and use a lot of heap memory
+    //        for their (child name lookup) and (block list) caches.
+    Vector<InodeIndex> unused_inodes;
+    for (auto& it : m_inode_cache) {
+        if (it.value->ref_count() != 1)
+            continue;
+        unused_inodes.append(it.key);
+    }
+    for (auto index : unused_inodes)
+        uncache_inode(index);
 }
 
 Ext2FSInode::Ext2FSInode(Ext2FS& fs, unsigned index)