selftests/bpf: update test_lwt_seg6local.sh according to iproute2

The shell file for test_lwt_seg6local contains an early iproute2 syntax
for installing a seg6local End.BPF route. iproute2 support for this
feature has recently been upstreamed, but with an additional keyword
required. This patch updates test_lwt_seg6local.sh to the definitive
iproute2 syntax

Signed-off-by: Mathieu Xhonneux <m.xhonneux@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>