KVM: Ensure local memslot copies operate on up-to-date arch-specific data

KVM: Ensure local memslot copies operate on up-to-date arch-specific data

When modifying memslots, snapshot the "old" memslot and copy it to the
"new" memslot's arch data after (re)acquiring slots_arch_lock.  x86 can
change a memslot's arch data while memslot updates are in-progress so
long as it holds slots_arch_lock, thus snapshotting a memslot without
holding the lock can result in the consumption of stale data.

Fixes: b10a038e84d1 ("KVM: mmu: Add slots_arch_lock for memslot arch fields")
Cc: stable@vger.kernel.org
Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211104002531.1176691-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>