rtnetlink: Define RTNL_FLAG_DOIT_PERNET for per-netns RTNL doit().

rtnetlink: Define RTNL_FLAG_DOIT_PERNET for per-netns RTNL doit().

We will push RTNL down to each doit() as rtnl_net_lock().

We can use RTNL_FLAG_DOIT_UNLOCKED to call doit() without RTNL, but doit()
will still hold RTNL.

Let's define RTNL_FLAG_DOIT_PERNET as an alias of RTNL_FLAG_DOIT_UNLOCKED.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>