zsmalloc: fix races between modifications of fullness and isolated

We encountered many kernel exceptions of VM_BUG_ON(zspage->isolated ==
0) in dec_zspage_isolation() and BUG_ON(!pages[1]) in zs_unmap_object()
lately.  This issue only occurs when migration and reclamation occur at
the same time.

With our memory stress test, we can reproduce this issue several times
a day.  We have no idea why no one else encountered this issue.  BTW,
we switched to the new kernel version with this defect a few months
ago.

Since fullness and isolated share the same unsigned int, modifications of
them should be protected by the same lock.

[[email protected]: move comment]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: c4549b871102 ("zsmalloc: remove zspage isolation for migration")
Signed-off-by: Andrew Yang <[email protected]>
Reviewed-by: Sergey Senozhatsky <[email protected]>
Cc: AngeloGioacchino Del Regno <[email protected]>
Cc: Matthias Brugger <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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