Don't bother masking off the non-queue state bits. It is not necessary
in non-INVARIANTS kernels, so just perform the mask in the assertions
Remove a comment about the order in which we set and clear flags. It
applies to an earlier version of the batched processing that I did, but
now it is only ever safe to test for or toggle PGA_ENQUEUED with the
page queue lock held.
Avoid unconditionally clearing both PGA_REQUEUE and PGA_REQUEUE_HEAD.
It is possible that another thread is concurrently setting
PGA_REQUEUE_HEAD and submitting a batch entry while
vm_pqbatch_process_page() is applying PGA_REQUEUE.