bareudp: fix NULL pointer dereference in bareudp_fill_metadata_dst()

bareudp_fill_metadata_dst() passes bareudp->sock to
udp_tunnel6_dst_lookup() in the IPv6 path without a NULL check.
The socket is only created in bareudp_open() and NULLed in
bareudp_stop(), so calling this function while the device is down
triggers a NULL dereference via sock->sk.

BUG: kernel NULL pointer dereference, address: 0000000000000018
RIP: 0010:udp_tunnel6_dst_lookup (net/ipv6/ip6_udp_tunnel.c:160)
Call Trace:
<TASK>
bareudp_fill_metadata_dst (drivers/net/bareudp.c:532)
do_execute_actions (net/openvswitch/actions.c:901)
ovs_execute_actions (net/openvswitch/actions.c:1589)
ovs_packet_cmd_execute (net/openvswitch/datapath.c:700)
genl_family_rcv_msg_doit (net/netlink/genetlink.c:1114)
genl_rcv_msg (net/netlink/genetlink.c:1209)
netlink_rcv_skb (net/netlink/af_netlink.c:2550)
</TASK>

Add a NULL check returning -ESHUTDOWN, consistent with the xmit paths
in the same driver.

Fixes: 571912c69f0e ("net: UDP tunnel encapsulation module for tunnelling different protocols like MPLS, IP, NSH etc.")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260426165350.1663137-2-bestswngs@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>