Btrfs: Add a lookup cache to the extent state tree

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index 6fd5e2c..160da3f 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -48,6 +48,7 @@
 	spinlock_t lru_lock;
 	struct list_head buffer_lru;
 	int lru_size;
+	struct extent_state *last;
 };
 
 struct extent_state {