mempool: ensure early_alloc_page errors are visible

calling debug() and then abort() buffers a message that never
has a chance to reach the console early on;
pass the message to display directly to abort() instead.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>