selftests/bpf: Test that dst is cleared on same-protocol encap

selftests/bpf: Test that dst is cleared on same-protocol encap

Verify that bpf_skb_adjust_room() clears the routing dst even when
the encap L3 protocol matches the original packet (e.g. IPIP).
The dst selected for the inner packet is not valid for the
encapsulated result; a stale dst could lead to misrouting.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://patch.msgid.link/20260329180428.2657785-2-kuba@kernel.org