Change BPF selftest test_xdp_vlan.sh to (default) use generic XDP.

This selftest was created together with a fix for generic XDP, in commit
29724956 ("net: fix generic XDP to handle if eth header was
mangled"). And was suppose to catch if generic XDP was broken again.

The tests are using veth and assumed that veth driver didn't support
native driver XDP, thus it used the (ip link set) 'xdp' attach that fell
back to generic-XDP. But veth gained native-XDP support in 948d4f21
("veth: Add driver XDP"), which caused this test script to use
native-XDP.

Fixes: 948d4f21 ("veth: Add driver XDP")
Fixes: 97396ff0 ("selftests/bpf: add XDP selftests for modifying and popping VLAN headers")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>