netpoll: Add locking for netpoll_setup/cleanup

As it stands, netpoll_setup and netpoll_cleanup have no locking
protection whatsoever.  So chaos ensures if two entities try to
perform them on the same device.

This patch adds RTNL to the equation.  The code has been rearranged so
that bits that do not need RTNL protection are now moved to the top of
netpoll_setup.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>