The btree functions currently don't take a specific root argument. They
assume, deep down in btree_walk, that there's only one btree in the
system. We're going to be adding a few more to support richer
allocation.
To prepare for this we have the btree functions take an explicit btree
argument. This should make no functional difference.
Signed-off-by: Zach Brown <zab@versity.com>