virtio-mem: Big Block Mode (BBM) - safe memory hotunplug

Let's add a safe mechanism to unplug memory, avoiding long/endless loops
when trying to offline memory - similar to in SBM.

Fake-offline all memory (via alloc_contig_range()) before trying to
offline+remove it. Use this mode as default, but allow to enable the other
mode explicitly (which could give better memory hotunplug guarantees in
some environments).

The "unsafe" mode can be enabled e.g., via virtio_mem.bbm_safe_unplug=0
on the cmdline.

Reviewed-by: Wei Yang <[email protected]>
Cc: "Michael S. Tsirkin" <[email protected]>
Cc: Jason Wang <[email protected]>
Cc: Pankaj Gupta <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Michael S. Tsirkin <[email protected]>

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