KVM: x86/mmu: Speed up slot_rmap_walk_next for sparsely populated rmaps

Avoid calling handlers on empty rmap entries and skip to the next non
empty rmap entry.

Empty rmap entries are noop in handlers.

Signed-off-by: Vipin Sharma <vipinsh@google.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220502220347.174664-1-vipinsh@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>