mempool: Don't store redundant information for k_malloc/k_free

We don't need to store the full k_mem_block, rather just the
k_mem_block_id. In effect, this saves 4 bytes of memory per allocated
memory chunk. Also take advantage of the newly introduced
k_mem_pool_free_id API here.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>