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 <idosch@idosch.org>
Fixes: 2c2493b9

 ("xfrm: lwtunnel: add lwtunnel support for xfrm interfaces in collect_md mode")
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>