vm_page: Tighten the object lock assertion in vm_page_invalid()

A page must not become invalid while vm_fault_soft_fast() is attempting
to map unbusied pages for reading.

Note that all callers hold the object write lock already, and
vm_page_set_invalid() asserts the object write lock.

Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33250