selftests: rtnetlink: use internal netns switch for ip commands

'ip' can switch network namespaces internally and then run a given
command relative to that namespace without the need to fork and exec
another ip instance. Update all references of the form:
    ip netns exec "$testns" ip ...
to
    ip -netns "$testns" ...

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>