x86/access: Make SMEP play nice with 5-level paging

Rework the walker to clear the USER bit on the test's text region when
enabling SMEP.  The walk_pte library function assumes 4-level paging
(see PAGE_LEVEL), and completely botches 5-level paging.  Through sheer
dumb luck, the test works with 5-level paging, likely because of an
unintentional collision with the test's own PTEs.

Punt on the library for the time being as the access test is obviously
more than capable of walking page tables, and fixing the library properly
will involve poking many more tests.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>