KVM: x86/mmu: Rename shadow MMU functions that deal with shadow pages

Rename 2 functions:

  kvm_mmu_get_page() -> kvm_mmu_get_shadow_page()
  kvm_mmu_free_page() -> kvm_mmu_free_shadow_page()

This change makes it clear that these functions deal with shadow pages
rather than struct pages. It also aligns these functions with the naming
scheme for kvm_mmu_find_shadow_page() and kvm_mmu_alloc_shadow_page().

Prefer "shadow_page" over the shorter "sp" since these are core
functions and the line lengths aren't terrible.

No functional change intended.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220516232138.1783324-9-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>