x86/HVM: fix setting mem access to default

commit 3b0bcb89 ("x86/mm/p2m: Move p2m code in HVMOP_[gs]et_mem_access
into p2m.c") introduced an off-by-one mistake forcing an input of
HVMMEM_access_default to always fail. Since related, also eliminate the
inefficient setup of an on-stack array for each function invocation.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -1342,25 +1342,32 @@ long p2m_set_mem_access(struct domain *d
     mfn_t mfn;
     long rc = 0;
 
-    /* N.B. _not_ static: initializer depends on p2m->default_access */
-    p2m_access_t memaccess[] = {
-        p2m_access_n,
-        p2m_access_r,
-        p2m_access_w,
-        p2m_access_rw,
-        p2m_access_x,
-        p2m_access_rx,
-        p2m_access_wx,
-        p2m_access_rwx,
-        p2m_access_rx2rw,
-        p2m_access_n2rwx,
-        p2m->default_access,
+    static const p2m_access_t memaccess[] = {
+#define ACCESS(ac) [HVMMEM_access_##ac] = p2m_access_##ac
+        ACCESS(n),
+        ACCESS(r),
+        ACCESS(w),
+        ACCESS(rw),
+        ACCESS(x),
+        ACCESS(rx),
+        ACCESS(wx),
+        ACCESS(rwx),
+        ACCESS(rx2rw),
+        ACCESS(n2rwx),
+#undef ACCESS
     };
 
-    if ( (unsigned) access >= HVMMEM_access_default )
+    switch ( access )
+    {
+    case 0 ... ARRAY_SIZE(memaccess) - 1:
+        a = memaccess[access];
+        break;
+    case HVMMEM_access_default:
+        a = p2m->default_access;
+        break;
+    default:
         return -EINVAL;
-
-    a = memaccess[access];
+    }
 
     /* If request to set default access */
     if ( pfn == ~0ul )