remove_one() was not incrementing the key for the beginning of the
range, so not all entries were being removed. This resulted in
discards that were not unmapping all blocks.
Fixes: 4ec331c3ea ("dm btree: add dm_btree_remove_leaves()") Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>