Fix handling of subpage BIO_WRITE and BIO_DELETE requests on swap MDs.

Such requests would previously mark the entire page as valid, which was
incorrect since nothing guaranteed that the page's contents had been
initialized. This change also modifies subpage BIO_DELETEs so that the
entire page is marked dirty, rather than only a subrange. There is no
benefit to creating partially dirty swap pages.

Reviewed by: alc, kib (previous version)
MFC after: 3 days