selftests/net: convert rtnetlink.sh to run it in unique namespace
When running the test in namespace, the debugfs may not load automatically.
So add a checking to make sure debugfs loaded. Here is the test result
after conversion.
# ./rtnetlink.sh
PASS: policy routing
PASS: route get
...
PASS: address proto IPv4
PASS: address proto IPv6
Acked-by: David Ahern <dsahern@kernel.org> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>