Kernel: Don't hog file system lock when doing BlockBasedFileSystem I/O

The file system lock is meant to protect the file system metadata
(super blocks, bitmaps, etc.) Not protect processes from reading
independent parts of the disk at once.

This patch introduces a new lock to protect the *block cache* instead,
which is the real thing that needs synchronization.

Kernel/FileSystem/BlockBasedFileSystem.h

@@ -42,6 +42,7 @@ private:
     DiskCache& cache() const;
     void flush_specific_block_if_needed(BlockIndex index);
 
+    mutable Lock m_cache_lock;
     mutable OwnPtr<DiskCache> m_cache;
 };