net: remove WARN_ON_ONCE() from the dev_fill_forward_path() loop check

ipip_fill_forward_path() and ip6_tnl_fill_forward_path() look up the
route to the tunnel's remote endpoint and set ctx->dev to its device,
which is the tunnel itself when that route resolves back to the tunnel.
dev_fill_forward_path() then makes no progress and trips
WARN_ON_ONCE(last_dev == ctx->dev) as soon as a flowtable tries to
offload a flow through the tunnel. That routing loop is a configuration
any CAP_NET_ADMIN user can set up, and ip_tunnel_xmit() and
ip6_tnl_xmit() already treat it as a tx error, so remove the warning and
just fail the walk, as commit 008e7a7c293b ("net: remove WARN_ON_ONCE
when accessing forward path array") did for the path stack overflow.

Fixes: ab427db17885 ("netfilter: flowtable: Add IPIP rx sw acceleration")
Fixes: d98103575dcd ("netfilter: flowtable: Add IP6IP6 rx sw acceleration")
Closes: https://lore.kernel.org/all/CA+0ovCgaRvbd0Udj70b2xxG8Cx3CaCpNhnf1V4RWQuDveZYZhA@mail.gmail.com/
Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Farhad Alemi <farhad.alemi@berkeley.edu>
Reviewed-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Link: https://patch.msgid.link/CA+0ovCgKDOk+Bg6Gh5Lwx94u_jJjQ30-vY1JcY2BYfhnWJJbPA@mail.gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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