selftests/bpf: Extend test for sockmap update with same

Verify that the sockmap link was not severed, and socket's entry is indeed
removed from the map when the corresponding descriptor gets closed.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241202-sockmap-replace-v1-2-1e88579e7bd5@rbox.co