virtio-balloon: Fix QEMU crashes on pagesize > BALLOON_PAGE_SIZE

virtio-balloon: Fix QEMU crashes on pagesize > BALLOON_PAGE_SIZE

We are using the wrong functions to set/clear bits, effectively touching
multiple bits, writing out of range of the bitmap, resulting in memory
corruptions. We have to use set_bit()/clear_bit() instead.

Can easily be reproduced by starting a qemu guest on hugetlbfs memory,
inflating the balloon. QEMU crashes. This never could have worked
properly - especially, also pages would have been discarded when the
first sub-page would be inflated (the whole bitmap would be set).

While testing I realized, that on hugetlbfs it is pretty much impossible
to discard a page - the guest just frees the 4k sub-pages in random order
most of the time. I was only able to discard a hugepage a handful of
times - so I hope that now works correctly.

Fixes: ed48c59875b6 ("virtio-balloon: Safely handle BALLOON_PAGE_SIZE < host page size")
Fixes: b27b32391404 ("virtio-balloon: Fix possible guest memory corruption with inflates & deflates")
Cc: [email protected] #v4.0.0
Acked-by: David Gibson <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
Message-Id: <20190722134108[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>