ocfs2: sparse b-tree support

Introduce tree rotations into the b-tree code. This will allow ocfs2 to
support sparse files. Much of the added code is designed to be generic (in
the ocfs2 sense) so that it can later be re-used to implement large
extended attributes.

This patch only adds the rotation code and does minimal updates to callers
of the extent api.

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

fs/ocfs2/ocfs2.h

@@ -303,6 +303,13 @@ static inline int ocfs2_should_order_data(struct inode *inode)
 	return 1;
 }
 
+static inline int ocfs2_sparse_alloc(struct ocfs2_super *osb)
+{
+	if (osb->s_feature_incompat & OCFS2_FEATURE_INCOMPAT_SPARSE_ALLOC)
+		return 1;
+	return 0;
+}
+
 /* set / clear functions because cluster events can make these happen
  * in parallel so we want the transitions to be atomic. this also
  * means that any future flags osb_flags must be protected by spinlock