Do grammar fix in the comment to record the right commit message for
r283162.

Fix a cosmetic issue with vm_page_alloc() calling vm_page_free_toq()
with the page not completely satisfying vm_page_free() assertions.
The page is not owned by the object, since insertion failed. But
besides m->object reset to NULL, we should also set VPO_UNMANAGED flag
for consistency.

Reported by: pho
Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week