selftests/bpf/xdp_redirect_multi: Limit the tests in netns

As I want to test both DEVMAP and DEVMAP_HASH in XDP multicast redirect, I
limited DEVMAP max entries to a small value for performace. When the test
runs after amount of interface creating/deleting tests. The interface index
will exceed the map max entries and xdp_redirect_multi will error out with
"Get interfacesInterface index to large".

Fix this issue by limit the tests in netns and specify the ifindex when
creating interfaces.

Fixes: d2329247 ("selftests/bpf: Add xdp_redirect_multi test")
Reported-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211027033553.962413-5-liuhangbin@gmail.com