KVM: x86: adjust kvm_mmu_page member to save 8 bytes

On a 64bits machine, struct is naturally aligned with 8 bytes. Since
kvm_mmu_page member *unsync* and *role* are less then 4 bytes, we can
rearrange the sequence to compace the struct.

As the comment shows, *role* and *gfn* are used to key the shadow page. In
order to keep the comment valid, this patch moves the *unsync* up and
exchange the position of *role* and *gfn*.

From /proc/slabinfo, it shows the size of kvm_mmu_page is 8 bytes less and
with one more object per slap after applying this patch.

    # name            <active_objs> <num_objs> <objsize> <objperslab>
    kvm_mmu_page_header      0           0       168         24

    kvm_mmu_page_header      0           0       160         25
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -281,18 +281,18 @@ struct kvm_rmap_head {
 struct kvm_mmu_page {
 	struct list_head link;
 	struct hlist_node hash_link;
+	bool unsync;
 
 	/*
 	 * The following two entries are used to key the shadow page in the
 	 * hash table.
 	 */
-	gfn_t gfn;
 	union kvm_mmu_page_role role;
+	gfn_t gfn;
 
 	u64 *spt;
 	/* hold the gfn of each spte inside spt */
 	gfn_t *gfns;
-	bool unsync;
 	int root_count;          /* Currently serving as active root */
 	unsigned int unsync_children;
 	struct kvm_rmap_head parent_ptes; /* rmap pointers to parent sptes */