dm btree: prefetch child nodes when walking tree for a dm_btree_del

dm-btree now takes advantage of dm-bufio's ability to prefetch data via
dm_bm_prefetch().  Prior to this change many btree node visits were
causing a synchronous read.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/persistent-data/dm-block-manager.h b/drivers/md/persistent-data/dm-block-manager.h
index be5bff6..9a82083 100644
--- a/drivers/md/persistent-data/dm-block-manager.h
+++ b/drivers/md/persistent-data/dm-block-manager.h
@@ -108,6 +108,11 @@
 int dm_bm_flush_and_unlock(struct dm_block_manager *bm,
 			   struct dm_block *superblock);
 
+ /*
+  * Request data be prefetched into the cache.
+  */
+void dm_bm_prefetch(struct dm_block_manager *bm, dm_block_t b);
+
 /*
  * Switches the bm to a read only mode.  Once read-only mode
  * has been entered the following functions will return -EPERM.