xen/kexec: return error code for unknown hypercalls

Currently do_kexec_op_internal() will return 0 for unknown hypercalls.  Fix
this by returning -EOPNOTSUPP instead.

While there remove the pointless initialization of ret to -EINVAL; it just
makes the code confusing.

Fixes: d046f361dc ("Xen Security Modules: XSM")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/common/kexec.c

@@ -1217,7 +1217,7 @@ static int do_kexec_op_internal(unsigned long op,
                                 XEN_GUEST_HANDLE_PARAM(void) uarg,
                                 bool compat)
 {
-    int ret = -EINVAL;
+    int ret;
 
     ret = xsm_kexec(XSM_PRIV);
     if ( ret )
@@ -1258,6 +1258,10 @@ static int do_kexec_op_internal(unsigned long op,
     case KEXEC_CMD_kexec_status:
         ret = kexec_status(uarg);
         break;
+
+    default:
+        ret = -EOPNOTSUPP;
+        break;
     }
 
     clear_bit(KEXEC_FLAG_IN_HYPERCALL, &kexec_flags);