KVM: introduce KVM_PFN_ERR_FAULT

After that, the exported and un-inline function, get_fault_pfn,
can be removed

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index aa9a987..9cf90c8 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2512,7 +2512,7 @@
 
 	slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn, no_dirty_log);
 	if (!slot)
-		return get_fault_pfn();
+		return KVM_PFN_ERR_FAULT;
 
 	hva = gfn_to_hva_memslot(slot, gfn);