KVM: x86/mmu: Verify shadow walk doesn't terminate early in page faults

WARN and bail if the shadow walk for faulting in a SPTE terminates early,
i.e. doesn't reach the expected level because the walk encountered a
terminal SPTE.  The shadow walks for page faults are subtle in that they
install non-leaf SPTEs (zapping leaf SPTEs if necessary!) in the loop
body, and consume the newly created non-leaf SPTE in the loop control,
e.g. __shadow_walk_next().  In other words, the walks guarantee that the
walk will stop if and only if the target level is reached by installing
non-leaf SPTEs to guarantee the walk remains valid.

Opportunistically use fault->goal-level instead of it.level in
FNAME(fetch) to further clarify that KVM always installs the leaf SPTE at
the target level.

Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20210906122547.263316-1-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>