void * mm_ceiling(void *bp, size_t asize)
      param: bp - pointer to a block that is in the tree of
      free blocks
      Description: finds and returns the block in the subtree
      at bp with the smallest size that is >= asize. If no such
      block exists, return NULL.

      Return: A pointer to a block with the smallest size that
      is >= asize. If no such block exists, returns NULL.