KVM: MMU: fix pointer cast

On a 32 bits compile, commit 3da0dd43
introduced the following warnings:

arch/x86/kvm/mmu.c: In function ‘kvm_set_pte_rmapp’:
arch/x86/kvm/mmu.c:770: warning: cast to pointer from integer of different size
arch/x86/kvm/mmu.c: In function ‘kvm_set_spte_hva’:
arch/x86/kvm/mmu.c:849: warning: cast from pointer to integer of different size

The following patch uses 'unsigned long' instead of u64 to match the
pointer size on both arches.

Signed-off-by: Frederik Deweerdt <frederik.deweerdt@xprog.eu>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>