Clear m->object for the page taken from the delayed free list for
reuse as the pv chink page in reclaim_pv_chunk().  Having non-NULL
m->object is wrong for page not owned by an object and confuses both
vm_page_free_toq() and vm_page_remove() when the page is freed later.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days