selftests/bpf: reduce time to execute test_xdp_vlan.sh

Given the increasing number of BPF selftests, it makes sense to
reduce the time to execute these tests.  The ping parameters are
adjusted to reduce the time from measures 9 sec to approx 2.8 sec.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>