selftests/tc-testing: Verify IFE can handle truncated inner Ethernet header

selftests/tc-testing: Verify IFE can handle truncated inner Ethernet header

Add a tdc test that exercises the act_ife decode path with a malformed
IFE packet whose encapsulated inner Ethernet header is truncated.

The injected frame has a valid outer Ethernet header (ethertype 0xED3E)
and a minimal IFE header (metalen 2, i.e. no metadata TLVs), but the
payload that should hold the original frame is a single byte instead of
a full Ethernet header. Once ife_decode() strips the outer header and
the IFE metadata, fewer than ETH_HLEN bytes are left, which previously
let eth_type_trans() read past the end of the linear data.

Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20260610183814.1648888-3-n05ec@lzu.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>