virtio-mem: use page_offline_(start|end) when setting PageOffline()

Let's properly use page_offline_(start|end) to synchronize setting
PageOffline(), so we won't have valid page access to unplugged memory
regions from /proc/kcore.

Existing balloon implementations usually allow reading inflated memory;
doing so might result in unnecessary overhead in the hypervisor, which is
currently the case with virtio-mem.

For future virtio-mem use cases, it will be different when using shmem,
huge pages, !anonymous private mappings, ...  as backing storage for a VM.
virtio-mem unplugged memory must no longer be accessed and access might
result in undefined behavior.  There will be a virtio spec extension to
document this change, including a new feature flag indicating the changed
behavior.  We really don't want to race against PFN walkers reading random
page content.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Acked-by: Michael S. Tsirkin <[email protected]>
Acked-by: Mike Rapoport <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Cc: Aili Yao <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Alex Shi <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: Jason Wang <[email protected]>
Cc: Jiri Bohac <[email protected]>
Cc: "K. Y. Srinivasan" <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: Steven Price <[email protected]>
Cc: Wei Liu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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