xen/mmu: Fix for linker errors when CONFIG_SMP is not defined.

Simple enough - we use an extern defined symbol which is not
defined when CONFIG_SMP is not defined. This fixes the linker
dying.

CC: stable@kernel.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 673e968..0ccccb6 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1232,7 +1232,11 @@
 {
 	struct {
 		struct mmuext_op op;
+#ifdef CONFIG_SMP
 		DECLARE_BITMAP(mask, num_processors);
+#else
+		DECLARE_BITMAP(mask, NR_CPUS);
+#endif
 	} *args;
 	struct multicall_space mcs;