block, drbd: use mempool_create_slab_pool()

Mempools created for slab caches should use
mempool_create_slab_pool().

Cc: Lars Ellenberg <drbd-dev@lists.linbit.com>
Cc: Jens Axboe <axboe@fb.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>