dm btree remove: fix bug in remove_one()

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 <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/persistent-data/dm-btree-remove.c

···
 					     value_ptr(n, index));
 
 		delete_at(n, index);
+		keys[last_level] = k + 1ull;
 
 	} else
 		r = -ENODATA;