fs: dlm: fix build with CONFIG_IPV6 disabled

This patch will surround the AF_INET6 case in sk_error_report() of dlm
with a #if IS_ENABLED(CONFIG_IPV6). The field sk->sk_v6_daddr is not
defined when CONFIG_IPV6 is disabled. If CONFIG_IPV6 is disabled, the
socket creation with AF_INET6 should already fail because a runtime
check if AF_INET6 is registered. However if there is the possibility
that AF_INET6 is set as sk_family the sk_error_report() callback will
print then an invalid family type error.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 4c3d905 ("fs: dlm: don't call kernel_getpeername() in error_report()")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>

fs/dlm/lowcomms.c

@@ -608,6 +608,7 @@ static void lowcomms_error_report(struct sock *sk)
 				   ntohs(inet->inet_dport), sk->sk_err,
 				   sk->sk_err_soft);
 		break;
+#if IS_ENABLED(CONFIG_IPV6)
 	case AF_INET6:
 		printk_ratelimited(KERN_ERR "dlm: node %d: socket error "
 				   "sending to node %d at %pI6c, "
@@ -616,6 +617,7 @@ static void lowcomms_error_report(struct sock *sk)
 				   ntohs(inet->inet_dport), sk->sk_err,
 				   sk->sk_err_soft);
 		break;
+#endif
 	default:
 		printk_ratelimited(KERN_ERR "dlm: node %d: socket error "
 				   "invalid socket family %d set, "