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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 39a7396f5d0c3eac4401229e1b24fb8b89efc862)