ixgb: remove ndo_poll_controller

As diagnosed by Song Liu, ndo_poll_controller() can
be very dangerous on loaded hosts, since the cpu
calling ndo_poll_controller() might steal all NAPI
contexts (for all RX/TX queues of the NIC). This capture
can last for unlimited amount of time, since one
cpu is generally not able to drain all the queues under load.

ixgb uses NAPI for TX completions, so we better let core
networking stack call the napi->poll() to avoid the capture.

This also removes a problematic use of disable_irq() in
a context it is forbidden, as explained in commit
af3e0fcf

 ("8139too: Use disable_irq_nosync() in
rtl8139_poll_controller()")

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>