vxlan: report a circular route as SKB_DROP_REASON_RECURSION_LIMIT

udp_tunnel_dst_lookup() and udp_tunnel6_dst_lookup() return -ELOOP
when the route to the remote end goes out of the vxlan device itself,
and vxlan_xmit_one() counts that in collisions, apart from the
-ENETUNREACH of a failed lookup. The drop reason does not tell them
apart: both are reported as SKB_DROP_REASON_IP_OUTNOROUTES, which
describes a failed route lookup.

Such a route would send the encapsulated packet back into the device
it leaves, which is the dead loop on a virtual device that
SKB_DROP_REASON_RECURSION_LIMIT describes. Report the -ELOOP case with
that reason, and add the tunnel case to the description of the reason
as an example. ip_tunnel and ip6_tunnel make the same check,
rt->dst.dev == dev in ip_tunnel_xmit() and ip_md_tunnel_xmit() and
tdev == dev in ip6_tnl_xmit(), and can report it with the same reason.

Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20261005201417.906344-5-littlesmilingcloud@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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