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: NJoe Thornber <ejt@redhat.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>