netpoll: remove IFF_IN_NETPOLL flag

V4: rebase to net-next-2.6

This patch removes the flag IFF_IN_NETPOLL, we don't need it any more since
we have netpoll_tx_running() now.

Signed-off-by: WANG Cong <amwang@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>