xfrm: lwtunnel: squelch kernel warning in case XFRM encap type is not available

Ido reported that a kernel warning [1] can be triggered from
user space when the kernel is compiled with CONFIG_MODULES=y and
CONFIG_XFRM=n when adding an xfrm encap type route, e.g:

$ ip route add 198.51.100.0/24 dev dummy1 encap xfrm if_id 1
Error: lwt encapsulation type not supported.

The reason for the warning is that the LWT infrastructure has an
autoloading feature which is meant only for encap types that don't
use a net device,  which is not the case in xfrm encap.

Mute this warning for xfrm encap as there's no encap module to autoload
in this case.

[1]
 WARNING: CPU: 3 PID: 2746262 at net/core/lwtunnel.c:57 lwtunnel_valid_encap_type+0x4f/0x120
[...]
 Call Trace:
  <TASK>
  rtm_to_fib_config+0x211/0x350
  inet_rtm_newroute+0x3a/0xa0
  rtnetlink_rcv_msg+0x154/0x3c0
  netlink_rcv_skb+0x49/0xf0
  netlink_unicast+0x22f/0x350
  netlink_sendmsg+0x208/0x440
  ____sys_sendmsg+0x21f/0x250
  ___sys_sendmsg+0x83/0xd0
  __sys_sendmsg+0x54/0xa0
  do_syscall_64+0x35/0x80
  entry_SYSCALL_64_after_hwframe+0x63/0xcd

Reported-by: Ido Schimmel <[email protected]>
Fixes: 2c2493b ("xfrm: lwtunnel: add lwtunnel support for xfrm interfaces in collect_md mode")
Signed-off-by: Eyal Birger <[email protected]>
Tested-by: Ido Schimmel <[email protected]>
Reviewed-by: Nikolay Aleksandrov <[email protected]>
Signed-off-by: Steffen Klassert <[email protected]>

Author: ebirger

net/core/lwtunnel.c

@@ -48,9 +48,11 @@ static const char *lwtunnel_encap_str(enum lwtunnel_encap_types encap_type)
 		return "RPL";
 	case LWTUNNEL_ENCAP_IOAM6:
 		return "IOAM6";
+	case LWTUNNEL_ENCAP_XFRM:
+		/* module autoload not supported for encap type */
+		return NULL;
 	case LWTUNNEL_ENCAP_IP6:
 	case LWTUNNEL_ENCAP_IP:
-	case LWTUNNEL_ENCAP_XFRM:
 	case LWTUNNEL_ENCAP_NONE:
 	case __LWTUNNEL_ENCAP_MAX:
 		/* should not have got here */