When waiting on PTE allocation, another thread could free the l2_dtable while
we're not looking at it.
Fix this by increasing l2->l2_occupancy before we try to alloc (and decrease
it if the allocation failed, or if another thread did a similar allocation).

Submitted by: Kohji Okuno <okuno.kohji@jp.panasonic.com>
MFC after: 1 week