KVM: pfncache: check the need for invalidation under read lock first

KVM: pfncache: check the need for invalidation under read lock first

Taking a write lock on a pfncache will be disruptive if the cache is
heavily used (which only requires a read lock). Hence, in the MMU notifier
callback, take read locks on caches to check for a match; only taking a
write lock to actually perform an invalidation (after a another check).

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
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Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>
v10:
 - New in this version.