net: fix "netpoll: Allow netpoll_setup/cleanup recursion"

Remove rtnl_unlock() which had no corresponding rtnl_lock().

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>