Revert "KVM: MMU: fix validation of mmio page fault"

This reverts commit 41e3025e, which
was commit 6f691251 upstream.

The fix is only needed after commit f8f55942

 ("KVM: MMU: fast
invalidate all mmio sptes"), included in Linux 3.11.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>