xen/p2m/debugfs: Fix potential pointer exception.

We could be referencing the last + 1 element of level_name[]
array which would cause a pointer exception, because of the
initial setup of lvl=4.

[v1: No need to do this for type_name, pointed out by Ian Campbell]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/p2m.c

@@ -786,7 +786,7 @@ EXPORT_SYMBOL_GPL(m2p_find_override_pfn);
 int p2m_dump_show(struct seq_file *m, void *v)
 {
 	static const char * const level_name[] = { "top", "middle",
-						"entry", "abnormal" };
+						"entry", "abnormal", "error"};
 	static const char * const type_name[] = { "identity", "missing",
 						"pfn", "abnormal"};
 #define TYPE_IDENTITY 0