Ensure that queue state is cleared when vm_page_dequeue() returns.

Per-page queue state is updated non-atomically, with either the page
lock or the page queue lock held. When vm_page_dequeue() is called
without the page lock, in rare cases a different thread may be
concurrently dequeuing the page with the pagequeue lock held. Because
of the non-atomic update, vm_page_dequeue() might return before queue
state is completely updated, which can lead to race conditions.

Restrict the vm_page_dequeue() interface so that it must be called
either with the page lock held or on a free page, and busy wait when
a different thread is concurrently updating queue state, which must
happen in a critical section.

While here, do some related cleanup: inline vm_page_dequeue_locked()
into its only caller and delete a prototype for the unimplemented
vm_page_requeue_locked(). Replace the volatile qualifier for "queue"
added in r333703 with explicit uses of atomic_load_8() where required.

Reported and tested by: pho
Reviewed by: alc
Differential Revision: https://reviews.freebsd.org/D15980