ocfs2: Cache extent records

The extent map code was ripped out earlier because of an inability to deal
with holes. This patch adds back a simpler caching scheme requiring far less
code.

Our old extent map caching was designed back when meta data block caching in
Ocfs2 didn't work very well, resulting in many disk reads. These days our
metadata caching is much better, resulting in no un-necessary disk reads. As
a result, extent caching doesn't have to be as fancy, nor does it have to
cache as many extents. Keeping the last 3 extents seen should be sufficient
to give us a small performance boost on some streaming workloads.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

fs/ocfs2/inode.h

@@ -26,6 +26,8 @@
 #ifndef OCFS2_INODE_H
 #define OCFS2_INODE_H
 
+#include "extent_map.h"
+
 /* OCFS2 Inode Private Data */
 struct ocfs2_inode_info
 {
@@ -63,6 +65,8 @@ struct ocfs2_inode_info
 
 	struct ocfs2_caching_info	ip_metadata_cache;
 
+	struct ocfs2_extent_map		ip_extent_map;
+
 	struct inode			vfs_inode;
 };