KVM: x86/mmu: Tag disallowed NX huge pages even if they're not tracked

Tag shadow pages that cannot be replaced with an NX huge page regardless
of whether or not zapping the page would allow KVM to immediately create
a huge page, e.g. because something else prevents creating a huge page.

I.e. track pages that are disallowed from being NX huge pages regardless
of whether or not the page could have been huge at the time of fault.
KVM currently tracks pages that were disallowed from being huge due to
the NX workaround if and only if the page could otherwise be huge.  But
that fails to handled the scenario where whatever restriction prevented
KVM from installing a huge page goes away, e.g. if dirty logging is
disabled, the host mapping level changes, etc...

Failure to tag shadow pages appropriately could theoretically lead to
false negatives, e.g. if a fetch fault requests a small page and thus
isn't tracked, and a read/write fault later requests a huge page, KVM
will not reject the huge page as it should.

To avoid yet another flag, initialize the list_head and use list_empty()
to determine whether or not a page is on the list of NX huge pages that
should be recovered.

Note, the TDP MMU accounting is still flawed as fixing the TDP MMU is
more involved due to mmu_lock being held for read.  This will be
addressed in a future commit.

Fixes: 5bcaf3e1 ("KVM: x86/mmu: Account NX huge page disallowed iff huge page was requested")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221019165618.927057-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>