From: "Martin J. Bligh" <mbligh@aracnet.com>,
      Guillaume Morin <guillaume@morinfr.org>

We need to clear the software dirty bit on the tail pages of a compound page
when freeing it up.

The tail pages can become dirtied by mmap'ing /dev/mem, and writing into
any clustered page group (that a driver might have created or whatever).

Plus it's better to run all these pages through the free_pages_check checks
anyway.