Fix a page leak in the md(4) swap I/O path.

r356147 removed a vm_page_activate() call, but this is required to
ensure that pages end up in the page queues in the first place.

Restore the pre-r356157 logic. Now, without the page lock, the
vm_page_active() check is racy, but this race is harmless.

Reviewed by: alc, kib
Reported and tested by: pho
Differential Revision: https://reviews.freebsd.org/D23024