KVM: x86/mmu: Drop 'shared' param from tdp_mmu_link_page()

Drop @shared from tdp_mmu_link_page() and hardcode it to work for
mmu_lock being held for read.  The helper has exactly one caller and
in all likelihood will only ever have exactly one caller.  Even if KVM
adds a path to install translations without an initiating page fault,
odds are very, very good that the path will just be a wrapper to the
"page fault" handler (both SNP and TDX RFCs propose patches to do
exactly that).

No functional change intended.

Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210810224554.2978735-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>