mempool: prevent across-CPU page stealing.

This addresses the reace which is mentioned in the comment. If the
thread doing add_page() is migrated before the preempt_lock is taken
it may feed the pool with a page coming from different CPU's page
buffer.

The fix is to embrace the body with migration-lock.
disable routing lock

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
java: add -XX:+UseMembar