[NETPOLL]: Revert two bogus cleanups that broke netconsole.

[NETPOLL]: Revert two bogus cleanups that broke netconsole.

Based upon a report by Andrew Morton and code analysis done
by Jarek Poplawski.

This reverts c6a22978e5d155ed5de9a802046f80fc08386a00 ("[NETPOLL]:
Kill NETPOLL_RX_DROP, set but never tested.")  and
5a623dc0a88e43dcd0609efe3b0a379ab3c74467 ("[NETPOLL]: Don't need
rx_flags.").

The rx_flags did get tested for zero vs. non-zero and therefore we do
need those tests and that code which sets NETPOLL_RX_DROP et al.

Signed-off-by: David S. Miller <davem@davemloft.net>