If the last page of the file is partially full and whole valid

If the last page of the file is partially full and whole valid
portion is invalidated, invalidate the whole page.  Otherwise,
partially valid page appears on a page queue, which is wrong.  This
could only happen for the last page, because only then buffer which
triggered invalidation could not cover the whole page.

Reported and tested by:	pho (previous version)
Reviewed by:	alc
Sponsored by:	The FreeBSD Foundation
Approved by:	re (delphij)
MFC after:	2 weeks