phonet: Don't hold RTNL for addr_doit().

Now only __dev_get_by_index() depends on RTNL in addr_doit().

Let's use dev_get_by_index_rcu() and register addr_doit() with
RTNL_FLAG_DOIT_UNLOCKED.

While at it, I changed phonet_rtnl_msg_handlers[]'s init to C99
style like other core networking code.

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