dm persistent data: set some btree fn parms const

Mark some constant parameters constant in some dm-btree functions.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --cc drivers/md/dm-thin-metadata.c
Simple merge

diff --cc drivers/md/persistent-data/dm-btree.h
Simple merge