mempool: Add k_mem_pool_free_id API

The k_mem_pool_free API has no use for the full k_mem_block struct. In
particular, it only needs the k_mem_block_id. Introduce a new API
which takes only this essential struct. This paves the way to
simplify & improve the k_malloc/k_free implementation a bit.

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