Kernel: Handle OOM in DiskCache when mounting Ext2 filesystems

Create the disk cache up front, so we can verify it succeeds.
Make the KBuffer allocation fail-able, so we can properly handle
failure when the user asks up to mount a Ext2 filesystem under
OOM conditions.

Kernel/FileSystem/BlockBasedFileSystem.h

@@ -15,6 +15,7 @@ public:
     TYPEDEF_DISTINCT_ORDERED_ID(u64, BlockIndex);
 
     virtual ~BlockBasedFileSystem() override;
+    virtual bool initialize() override;
 
     u64 logical_block_size() const { return m_logical_block_size; };