KVM: PPC: Book3S HV: Fix refcounting of hugepages

The H_REGISTER_VPA hcall implementation in HV Power KVM needs to pin some
guest memory pages into host memory so that they can be safely accessed
from usermode.  It does this used get_user_pages_fast().  When the VPA is
unregistered, or the VCPUs are cleaned up, these pages are released using
put_page().

However, the get_user_pages() is invoked on the specific memory are of the
VPA which could lie within hugepages.  In case the pinned page is huge,
we explicitly find the head page of the compound page before calling
put_page() on it.

At least with the latest kernel, this is not correct.  put_page() already
handles finding the correct head page of a compound, and also deals with
various counts on the individual tail page which are important for
transparent huge pages.  We don't support transparent hugepages on Power,
but even so, bypassing this count maintenance can lead (when the VM ends)
to a hugepage being released back to the pool with a non-zero mapcount on
one of the tail pages.  This can then lead to a bad_page() when the page
is released from the hugepage pool.

This removes the explicit compound_head() call to correct this bug.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>