selftests/bpf: More compatible nc options in test_lwt_ip_encap

Out of the three nc implementations widely in use, at least two (BSD netcat
and nmap-ncat) do not support -l combined with -s. Modify the nc invocation
to be accepted by all of them.

Fixes: 17a90a78

 ("selftests/bpf: test that GSO works in lwt_ip_encap")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/9f177682c387f3f943bb64d849e6c6774df3c5b4.1570539863.git.jbenc@redhat.com