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>