KVM: x86/mmu: Pass const memslot to rmap_add()

Constify rmap_add()'s @slot parameter; it is simply passed on to
gfn_to_rmap(), which takes a const memslot.

No functional change intended.

Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220516232138.1783324-15-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>